tg.sh: include bad ref in ref_exists_rev failure message

[topgit/pro.git] / README

=========================================
TopGit -- A different patch queue manager
=========================================


DESCRIPTION
-----------

TopGit aims to make handling of large amounts of interdependent topic
branches easier. In fact, it is designed especially for the case where
you maintain a queue of third-party patches on top of another (perhaps
Git-controlled) project and want to easily organize, maintain and submit
them -- TopGit achieves that by keeping a separate topic branch for each
patch and providing some tools to maintain the branches.

See also:

        :REQUIREMENTS_:     Installation requirements
        :USAGE_:            Command Line details
        :`NO UNDO`_:        Where's the undo!!!
        :CONVENTIONS_:      Suggestions for organizing your TopGit branches
        :`EXTRA SETTINGS`_: Various possible "topgit.*" config settings
        :ALIASES_:          Git-like TopGit command aliases
        :GLOSSARY_:         All the TopGit vocabulary in one place
        :TECHNICAL_:        How it works behind the scenes


REQUIREMENTS
------------

TopGit is a collection of POSIX shell scripts so a POSIX-compliant shell is
required along with some standard POSIX-compliant utilities (e.g. sed, awk,
cat, etc.).  Git version 1.8.5 or later is also required.

To use TopGit with linked working trees (the ``git worktree add`` command),
at least Git version 2.5.0 (obviously, since that's when the ``git worktree``
command first appeared) is needed in which case linked working trees are then
fully supported for use with TopGit.

The scripts need to be preprocessed and installed and currently the Makefile
that does this requires GNU make, but that's an install-time-only requirement.

It is possible, however, to use the DESTDIR functionality to install TopGit to
a staging area on one machine, archive that and then unarchive it on another
machine to perform an install (provided the build prefix and other options are
compatible with the final installed location).


INSTALLATION
------------

See the file ``INSTALL``.


GIT REPOSITORY
--------------

The TopGit git repository can be found at <http://repo.or.cz/topgit/pro>.


RATIONALE
---------

Why not use something like StGIT or Guilt or ``rebase -i`` for maintaining
your patch queue?  The advantage of these tools is their simplicity;
they work with patch *series* and defer to the reflog facility for
version control of patches (reordering of patches is not
version-controlled at all).  But there are several disadvantages -- for
one, these tools (especially StGIT) do not actually fit well with plain
Git at all: it is basically impossible to take advantage of the index
effectively when using StGIT.  But more importantly, these tools
horribly fail in the face of a distributed environment.

TopGit has been designed around three main tenets:

        (i) TopGit is as thin a layer on top of Git as possible.  You
        still maintain your index and commit using Git; TopGit will only
        automate a few indispensable tasks.

        (ii) TopGit is anxious about *keeping* your history.  It will
        never rewrite your history, and all metadata is also tracked
        by Git, smoothly and non-obnoxiously.  It is good to have a
        *single* point when the history is cleaned up, and that is at
        the point of inclusion in the upstream project; locally, you
        can see how your patch has evolved and easily return to older
        versions.

        (iii) TopGit is specifically designed to work in a
        distributed environment.  You can have several instances of
        TopGit-aware repositories and smoothly keep them all
        up-to-date and transfer your changes between them.

As mentioned above, the main intended use-case for TopGit is tracking
third-party patches, where each patch is effectively a single topic
branch.  In order to flexibly accommodate even complex scenarios when
you track many patches where many are independent but some depend on
others, TopGit ignores the ancient Quilt heritage of patch series and
instead allows the patches to freely form graphs (DAGs just like Git
history itself, only "one level higher").  For now, you have to manually
specify which patches the current one depends on, but TopGit might help
you with that in the future in a darcs-like fashion.

A glossary_ plug: The union (i.e. merge) of patch dependencies is called
a *base* of the patch (topic branch).

Of course, TopGit is perhaps not the right tool for you:

        (i) TopGit is not complicated, but StGIT et al. are somewhat
        simpler, conceptually.  If you just want to make a linear
        purely-local patch queue, deferring to StGIT instead might
        make more sense.

        (ii) When using TopGit, your history can get a little hairy
        over time, especially with all the merges rippling through.
        ;-)


SYNOPSIS
--------

::

        ## Create and evolve a topic branch
        $ tg create t/gitweb/pathinfo-action
        tg: Automatically marking dependency on master
        tg: Creating t/gitweb/pathinfo-action base from master...
        $ ..hack..
        $ git commit
        $ ..fix a mistake..
        $ git commit

        ## Create another topic branch on top of the former one
        $ tg create t/gitweb/nifty-links
        tg: Automatically marking dependency on t/gitweb/pathinfo-action
        tg: Creating t/gitweb/nifty-links base from t/gitweb/pathinfo-action...
        $ ..hack..
        $ git commit

        ## Create another topic branch on top of master and submit
        ## the resulting patch upstream
        $ tg create t/revlist/author-fixed master
        tg: Creating t/revlist/author-fixed base from master...
        $ ..hack..
        $ git commit
        $ tg patch -m
        tg: Sent t/revlist/author-fixed
        From: pasky@suse.cz
        To: git@vger.kernel.org
        Cc: gitster@pobox.com
        Subject: [PATCH] Fix broken revlist --author when --fixed-string

        ## Create another topic branch depending on two others non-trivially
        $ tg create t/whatever t/revlist/author-fixed t/gitweb/nifty-links
        tg: Creating t/whatever base from t/revlist/author-fixed...
        tg: Merging t/whatever base with t/gitweb/nifty-links...
        Merge failed!
        tg: Please commit merge resolution and call: tg update --continue
        tg: It is also safe to abort this operation using `git reset --hard`
        tg: but please remember you are on the base branch now;
        tg: you will want to switch to a different branch.
        $ ..resolve..
        $ git commit
        $ tg update --continue
        $ ..hack..
        $ git commit

        ## Update a single topic branch and propagate the changes to
        ## a different one
        $ git checkout t/gitweb/nifty-links
        $ ..hack..
        $ git commit
        $ git checkout t/whatever
        $ tg info
        Topic Branch: t/whatever (1 commit)
        Subject: [PATCH] Whatever patch
        Base: 3f47ebc1
        Depends: t/revlist/author-fixed t/gitweb/nifty-links
        Needs update from:
                t/gitweb/nifty-links (1 commit)
        $ tg update
        tg: Updating base with t/gitweb/nifty-links changes...
        Merge failed!
        tg: Please commit merge resolution and call `tg update --continue`
        tg: (use `tg status` to see more options)
        $ ..resolve..
        $ git commit
        $ tg update --continue
        tg: Updating t/whatever against new base...
        Merge failed!
        tg: Please commit merge resolution and call `tg update --continue`
        tg: (use `tg status` to see more options)
        $ ..resolve..
        $ git commit
        $ tg update --continue

        ## Update a single topic branch and propagate the changes
        ## further through the dependency chain
        $ git checkout t/gitweb/pathinfo-action
        $ ..hack..
        $ git commit
        $ git checkout t/whatever
        $ tg info
        Topic Branch: t/whatever (1/2 commits)
        Subject: [PATCH] Whatever patch
        Base: 0ab2c9b3
        Depends: t/revlist/author-fixed t/gitweb/nifty-links
        Needs update from:
                t/gitweb/pathinfo-action (<= t/gitweb/nifty-links) (1 commit)
        $ tg update
        tg: Recursing to t/gitweb/nifty-links...
        [t/gitweb/nifty-links] tg: Updating base with t/gitweb/pathinfo-action changes...
        Merge failed!
        [t/gitweb/nifty-links] tg: Please commit merge resolution and call `tg update --continue`
        [t/gitweb/nifty-links] tg: (use `tg status` to see more options)
        $ ..resolve..
        $ git commit
        $ tg update --continue
        [t/gitweb/nifty-links] tg: Updating t/gitweb/nifty-links against new base...
        Merge failed!
        [t/gitweb/nifty-links] tg: Please commit merge resolution and call `tg update --continue`
        [t/gitweb/nifty-links] tg: (use `tg status` to see more options)
        $ ..resolve..
        $ git commit
        $ tg update --continue
        tg: Updating base with t/gitweb/nifty-links changes...
        tg: Updating t/whatever against new base...

        ## Clone a TopGit-controlled repository
        $ git clone URL repo
        $ cd repo
        $ tg remote --populate origin
        ...
        $ git fetch
        $ tg update

        ## Add a TopGit remote to a repository and push to it
        $ git remote add foo URL
        $ tg remote foo
        $ tg push -r foo

        ## Update from a non-default TopGit remote
        $ git fetch foo
        $ tg -r foo summary
        $ tg -r foo update


CONVENTIONS
-----------

When using TopGit there are several common conventions used when working with
TopGit branches.  None of them are enforced, they are only suggestions.

There are three typical uses for a TopGit branch:

    1. [PATCH]
       Normal TopGit branches that represent a single patch.  These are known
       as "patch" TopGit branches.
    2. [BASE]
       Empty TopGit branches with no dependencies (an empty ``.topdeps`` file)
       that represent a base upon which other "normal" TopGit branches depend.
       These are known as "base" TopGit branches (not to be confused with
       the refs/top-bases/... refs).
    3. [STAGE]
       Empty TopGit branches that serve as a staging area to bring together
       several other TopGit branches into one place so they can be used/tested
       all together.  These are known as "stage" TopGit branches.

An "empty" TopGit branch is one that does not have any changes of its own --
it may still have dependencies though ("stage" branches do, "base" branches do
not).  The ``tg summary`` output shows empty branches with a ``0`` in the
listing.  Normal "patch" branches that have not been annihilated, "base" and
"stage" branches fall into this category.  (Annihilated branches are normally
omitted from the ``tg summary`` output but can be shown if given explicitly as
an argument to the ``tg summary`` command.  However, the message line will be
incorrect since an annihilated branch has no ``.topmsg`` file of its own.)

A "patch" branch name typically starts with ``t/`` whereas "base" and "stage"
branch names often do not.

A "base" branch is created by using the ``--base`` option of ``tg create``
(aka ``--no-deps``) which will automatically suggest a "[BASE]" message prefix
rather than "[PATCH]".  A "stage" branch is created like a normal patch branch
except that the only changes that will ever be made to it are typically to
add/remove dependencies.  Its subject prefix must be manually changed to
"[STAGE]" to reflect its purpose.

Since both "base" and "stage" branches typically only have a use for the
"Subject:" line from their ``.topmsg`` file, they are quite easily created
using the ``--topmsg`` option of ``tg create``.

Use of "stage" and "base" branches is completely optional.  However, without
use of a "stage" branch it will be difficult to test multiple independent
patches together all at once.  A "base" branch is merely a convenience that
provides more explicit control over when a common base for a set of patches
gets updated as well as providing a branch that shows in ``tg summary`` output
and participates in ``tg remote --populate`` setup.

Another advantage to using a "stage" branch is that if a new "patch" branch
is created remotely and that new branch is added to a pre-existing "stage"
branch on the remote then when the local version of the "stage" branch is
updated (after fetching remote updates of course), that new dependency will
be merged into the local "stage" branch and the local version of the new remote
"patch" branch will be automatically set up at "tg update" time.

When using the ``tg tag`` command to create tags that record the current state
of one or more TopGit branches, the tags are often created with a name that
starts with ``t/``.

One last thing, you have enabled ``git rerere`` haven't you?


NO UNDO
-------

Beware, there is no "undo" after running a ``tg update``!

Well, that's not entirely correct.  Since ``tg update`` never discards commits
an "undo" operation is technically feasible provided the old values of all the
refs that were affected by the ``tg update`` operation can be determined and
then they are simply changed back to their previous values.

In practice though, it can be extremely tedious and error prone looking through
log information to try and determine what the correct previous values were.
Although, since TopGit tries to make sure reflogs are enabled for top-bases
refs, using Git's ``@{date}`` notation on all the refs dumped out by a
``tg tag --refs foo``, where "foo" is the branch that was updated whose update
needs to be undone, may work.

Alternatively, ``tg tag --stash`` can be used prior to the update and then
``tg revert`` used after the update to restore the previous state.  This
assumes, of course, that you remember to run ``tg tag --stash`` first.

The ``tg update`` command understands a ``--stash`` option that tells it to
automatically run ``tg tag --stash`` before it starts making changes (if
everything is up-to-date it won't run the stash command at all).

The ``--stash`` option is the default nowadays when running ``tg update``,
add the ``--no-stash`` option to turn it off.

There is a preference for this.  Setting the config value ``topgit.autostash``
to ``false`` will implicitly add the ``--no-stash`` option to any ``tg update``
command unless an explicit ``--stash`` option is given.

If you are likely to ever want to undo a ``tg update``, setting
``topgit.autostash`` to ``false`` is highly discouraged!

Note that the tags saved by ``tg tag --stash`` are stored in the
``refs/tgstash`` ref and its reflog.  Unfortunately, while Git is happy to
maintain the reflog (once it's been enabled which ``tg tag`` guarantees for
``refs/tgstash``), Git is unable to view an annotated/signed tag's reflog!
Instead Git dereferences the tag and shows the wrong thing.  Use the
``tg tag -g`` command to view the ``refs/tgstash`` reflog instead.


EXTRA SETTINGS
--------------

TopGit supports various config settings:

        :ALIASES_:              ``topgit.alias.*`` for Git-like command aliases
        :SEQUESTRATION_:        ``topgit.sequester`` for sequestration control
        :`tg migrate-bases`_:   ``topgit.top-bases`` for refs bases location
        :`tg update`_:          ``topgit.autostash`` automatic stash control
        :`REMOTE HANDLING`_:    ``topgit.remote`` TopGit's default remote


ALIASES
-------

These work exactly like Git's aliases except they are stored under
``topgit.alias.*`` instead.  See the ``git help config`` output under
the ``alias.*`` section for details.  Do note that while alias nesting is
explicitly permitted, a maximum nesting depth of 10 is enforced to help
detect accidental aliasing loops from wedging the machine.


SEQUESTRATION
-------------

No, this is not a section about budget nonsense.  ;)

TopGit keeps its metadata in ``.topdeps`` and ``.topmsg`` files.  In an effort
to facilitate cherry-picking and other Git activities on the patch changes
themselves while ignoring the TopGit metadata, TopGit attempts to keep all
changes to ``.topdeps`` and ``.topmsg`` files limited to commits that do NOT
contain changes to any other files.

This is a departure from previous TopGit versions that made no such effort.

Primarily this affects ``tg create`` and ``tg import`` (which makes use of
``tg create``) as ``tg create`` will commit the initial versions of
``.topdeps`` and ``.topmsg`` for a new TopGit-controlled branch in their own
commit instead of mixing them in with changes to other files.

The ``pre-commit`` hook will also attempt to separate out any ``.topdeps`` and
``.topmsg`` changes from commits that include changes to other files.

It is possible to defeat these checks without much effort (``pre-commit`` hooks
can easily be bypassed, ``tg create`` has a ``--no-commit`` option, many Git
commands simply do not run the ``pre-commit`` hook, etc.).

If you really, really, really, really want to change the default back to the
old behavior of previous TopGit versions where no such sequestration took
place, then set the ``topgit.sequester`` config variable explicitly to the
value ``false``.  But this is not recommended.


AMENDING AND REBASING AND UPDATE-REF'ING
----------------------------------------

In a word, "don't".

It is okay to manually update a top-bases/... ref when a) it has no depedencies
(i.e. it was created with the ``tg create`` ``--base`` option) and b) the
old top-bases/... ref value can be fast-forwarded to the new top-bases/...
value OR the new value contains ALL of the changes in the old value through
some other mechanism (perhaps they were cherry-picked or otherwise applied to
the new top-bases/... ref).  The same rules apply to non-TopGit-controlled
dependencies.  Use the ``tg update --base <branch> <new-ref>`` command to
safely make such an update while making it easy to set the merge commit
message at the same time.

Ignoring this rule and proceeding anyway with a non-fast-forward update to a
top-bases/... ref will result in changes present in the new value being merged
into the branch (at ``tg update`` time) as expected (possibly with conflicts),
but any changes that were contained in the old version of the top-bases/... ref
which have been dropped (i.e. are NOT contained in the new version of the
top-bases/... ref) will continue to be present in the branch!  To get rid of
the dropped commits, one or more "revert" commits will have to be manually
applied to the tip of the new top-bases/... value (which will then be merged
into the branch at next ``tg update`` time).

The only time it's safe to amend, rebase, filter or otherwise rewrite commits
contained in a TopGit controlled branch or non-TopGit branch is when those
commits are NOT reachable via any other ref!

Furthermore, while it is safe to rewrite merge commits (provided they meet the
same conditions) the merge commits themselves and the branches they are merging
in must be preserved during the rewrite and that can be rather tricky to get
right so it's not recommended.

For example, if, while working on a TopGit-controlled branch ``foo``, a bad
typo is noticed, it's okay to ammend/rebase to fix that provided neither
``tg update`` nor ``tg create`` has already been used to cause some other ref
to be able to reach the commit with the typo.

If an amend or rerwite is done anyway even though the commit with the typo is
reachable from some other ref, the typo won't really be removed.  What will
happen instead is that the new version without the typo will ultimately be
merged into the other ref(s) (at ``tg update`` time) likely causing a conflict
that will have to be manually resolved and the commit with the typo will
continue to be reachable from those other refs!

Instead just make a new commit to fix the typo.  The end result will end up
being the same but without the merge conflicts.

See also the discussion in the `NO UNDO`_ section.


SPEED AND CACHING
-----------------

TopGit needs to check many things to determine whether a TopGit branch is
up-to-date or not.  This can involve a LOT of git commands for a complex
dependency tree.  In order to speed things up, TopGit keeps a cache of results
in a ``tg-cache`` subdirectory in the ``.git`` directory.

Results are tagged with the original hash values used to get that result so
that items which have not been changed return their results quickly and items
which have been changed compute their new result and cache it for future use.

The ``.git/tg-cache`` directory may be removed at any time and the cache will
simply be recreated in an on-demand fashion as needed, at some speed penalty,
until it's fully rebuilt.

To force the cache to be fully pre-loaded, run the ``tg summary`` command
without any arguments.  Otherwise, normal day-to-day TopGit operations should
keep it more-or-less up-to-date.

While each TopGit command is running, it uses a temporary subdirectory also
located in the ``.git`` directory.  These directories are named
``tg-tmp.XXXXXX`` where the ``XXXXXX`` part will be random letters and digits.

These temporary directories should always be removed automatically after each
TopGit command finishes running.  As long as you are not in a subshell as a
result of a TopGit command stopping and waiting for a manual merge resolution,
it's safe to remove any of these directories that may have somehow accidentally
been left behind as a result of some failure that occurred while running a
TopGit command (provided, of course, it's not actually being used by a TopGit
command currently running in another terminal window or by another user on the
same repository).


USAGE
-----
``tg [-C <dir>] [-r <remote> | -u] [-c <name>=<val>] <subcommand> [<subcommand option/argument>...]``

        -C <dir>        Change directory to <dir> before doing anything
        -r <remote>     Pretend ``topgit.remote`` is set to <remote>
        -u              Pretend ``topgit.remote`` is not set
        -c <name=val>   Pass config option to git, may be repeated

The ``tg`` tool has several subcommands:

        :`tg annihilate`_:  Mark a TopGit-controlled branch as defunct
        :`tg base`_:        Show base commit for one or more TopGit branches
        :`tg checkout`_:    Shortcut for git checkout with name matching
        :`tg contains`_:    Which TopGit-controlled branch contains the commit
        :`tg create`_:      Create a new TopGit-controlled branch
        :`tg delete`_:      Delete a TopGit-controlled branch cleanly
        :`tg depend`_:      Add a new dependency to a TopGit-controlled branch
        :`tg export`_:      Export TopGit branch patches to files or a branch
        :`tg files`_:       Show files changed by a TopGit branch
        :`tg help`_:        Show TopGit help optionally using a browser
        :`tg import`_:      Import commit(s) to separate TopGit branches
        :`tg info`_:        Show status information about a TopGit branch
        :`tg log`_:         Run git log limiting revisions to a TopGit branch
        :`tg mail`_:        Shortcut for git send-email with ``tg patch`` output
        :`tg migrate-bases`_: Transition top-bases to new location
        :`tg next`_:        Show branches directly depending on a TopGit branch
        :`tg patch`_:       Generate a patch file for a TopGit branch
        :`tg prev`_:        Show non-annihilated TopGit dependencies for a branch
        :`tg push`_:        Run git push on TopGit branch(es) and depedencies
        :`tg rebase`_:      Auto continue git rebase if rerere resolves conflicts
        :`tg remote`_:      Set up remote for fetching/pushing TopGit branches
        :`tg revert`_:      Revert ref(s) to a state stored in a ``tg tag``
        :`tg status`_:      Show current TopGit status (e.g. in-progress update)
        :`tg summary`_:     Show various information about TopGit branches
        :`tg tag`_:         Create tag that records current TopGit branch state
        :`tg update`_:      Update TopGit branch(es) with respect to dependencies

tg help
~~~~~~~
        Our sophisticated integrated help facility.  Mostly duplicates
        what is below::

         # to list commands:
         $ tg help
         # to get help for a particular command:
         $ tg help <command>
         # to get help for a particular command in a browser window:
         $ tg help -w <command>
         # to get help on TopGit itself
         $ tg help tg
         # to get help on TopGit itself in a browser
         $ tg help -w tg

tg status
~~~~~~~~~
        Our sophisticated status facility.  Similar to Git's status command
        but shows any in-progress update that's awaiting a merge resolution
        or any other on-going TopGit activity (such as a branch creation).

        With a single ``--verbose`` (or ``-v``) option include a short status
        display for any dirty (but not untracked) files.  This also causes all
        non file status lines to be prefixed with "## ".

        With two (or more) ``--verbose`` (or ``-v``) options, additionally
        show full symbolic ref names and unabbreviated hash values.

        With the ``--exit-code`` option the exit code will be non-zero if any
        TopGit or Git operation is currently in progress or the working
        tree is unclean.

tg create
~~~~~~~~~
        Create a new TopGit-controlled topic branch of the given name
        (required argument) and switch to it.  If no dependencies are
        specified (by extra arguments passed after the first one), the
        current branch is assumed to be the only dependency.

        By default ``tg create`` opens an editor on the new ``.topmsg`` file
        and then commits the new ``.topmsg`` and ``.topdeps`` files
        automatically with a suitable default commit message.

        The commit message can be changed with the ``-m`` (or ``--message``) or
        ``-F`` (or ``--file``) option.  The automatic commit can be suppressed
        by using the ``--no-ccmmit`` option.  Running the editor on the new
        ``.topmsg`` file can be suppressed by using ``-n`` (or ``--no-edit``)
        (which also suppresses the automatic commit) or by providing an
        explicit value for the new ``.topmsg`` file using the ``--topmsg`` or
        ``--topmsg-file`` option.  In any case the ``.topmsg`` content will be
        automatically reformated to have a ``Subject:`` header line if needed.

        If more than one dependency is listed, the automatic commit will not
        take place until AFTER all the listed dependencies have been merged
        into a base commit which will require some manual merge resolutions if
        conflicts occur during the merge operations.

        Previous versions of TopGit behaved as though the ``--no-edit`` option
        was always given on the command line.

        The default behavior has been changed to promote a separation between
        commits that modify ``.topmsg`` and/or ``.topdeps`` and commits that
        modify other files.  This facilitates cleaner cherry picking and other
        patch maintenance activities.

        You should edit the patch description (contained in the ``.topmsg``
        file) as appropriate.  It will already contain some prefilled bits.
        You can set the ``topgit.to``, ``topgit.cc`` and ``topgit.bcc``
        git configuration variables (see ``man git-config``) in order to
        have ``tg create`` add these headers with the given default values
        to ``.topmsg`` before invoking the editor.

        The main task of ``tg create`` is to set up the topic branch base
        from the dependencies.  This may fail due to merge conflicts if more
        than one dependency is given.   In that case, after you commit the
        conflict resolution, you should call ``tg update --continue`` to
        finish merging the dependencies into the new topic branch base.

        With the ``--base`` (aka ``--no-deps``) option at most one dependency
        may be listed which may be any valid committish (instead of just
        refs/heads/...) and the newly created TopGit-controlled branch will
        have an empty ``.topdeps`` file.  This may be desirable in order to
        create a TopGit-controlled branch that has no changes of its own and
        serves merely to mark the common dependency that all other
        TopGit-controlled branches in some set of TopGit-controlled branches
        depend on.  A plain, non-TopGit-controlled branch can be used for the
        same purpose, but the advantage of a TopGit-controlled branch with no
        dependencies is that it will be pushed with ``tg push``, it will show
        up in the ``tg summary`` and ``tg info`` output with the subject from
        its ``.topmsg`` file thereby documenting what it's for and finally it
        can be set up with ``tg create -r`` and/or ``tg remote --populate`` to
        facilitate sharing.

        For example, ``tg create --base release v2.1`` will create a TopGit-
        controlled ``release`` branch based off the ``v2.1`` tag that can then
        be used as a base for creation of other TopGit-controlled branches.
        Then when the time comes to move the base for an entire set of changes
        up to ``v2.2`` the command ``tg update --base release v2.2`` can be
        used followed by ``tg update --all``.

        Using ``--base`` it's also possible to use ``tg create`` on an
        unborn branch (omit the dependency name or specify ``HEAD``).  The
        unborn branch itself can be made into the new TopGit branch (rather
        than being born empty and then having the new TopGit branch based off
        that) by specifying ``HEAD`` as the new branch's name (which is
        probably what you normally want to do in this case anyway so you can
        just run ``tg create --base HEAD`` to accomplish that).

        In an alternative use case, if ``-r <branch>`` is given instead of a
        dependency list, the topic branch is created based on the given
        remote branch.  With just ``-r`` the remote branch name is assumed
        to be the same as the local topic branch being created.  Since no
        new commits are created in this mode (only two refs will be updated)
        the editor will never be run for this use case.  Note that no other
        options may be combined with ``-r``.

        The ``--quiet`` (or ``-q``) option suppresses most informational
        messages.

tg delete
~~~~~~~~~
        Remove a TopGit-controlled topic branch of the given name
        (required argument). Normally, this command will remove only an
        empty branch (base == head) without dependendents; use ``-f`` to
        remove a non-empty branch or a branch that is depended upon by
        another branch.

        The ``-f`` option is also useful to force removal of a branch's
        base, if you used ``git branch -D B`` to remove branch B, and then
        certain TopGit commands complain, because the base of branch B
        is still there.

        Normally ``tg delete`` will refuse to delete the current branch.
        However, giving ``-f`` twice (or more) will force it to do so but it
        will first detach your HEAD.

        IMPORTANT: Currently, this command will *NOT* remove the branch
        from the dependency list in other branches. You need to take
        care of this *manually*.  This is even more complicated in
        combination with ``-f`` -- in that case, you need to manually
        unmerge the removed branch's changes from the branches depending
        on it.

        See also ``tg annihilate``.

        | TODO: ``-a`` to delete all empty branches, depfix, revert

tg annihilate
~~~~~~~~~~~~~
        Make a commit on the current TopGit-controlled topic branch
        that makes it equal to its base, including the presence or
        absence of .topmsg and .topdeps.  Annihilated branches are not
        displayed by ``tg summary``, so they effectively get out of your
        way.  However, the branch still exists, and ``tg push`` will
        push it (except if given the ``-a`` option).  This way, you can
        communicate that the branch is no longer wanted.

        When annihilating a branch that has dependents (i.e. branches
        that depend on it), those dependents have the dependencies of
        the branch being annihilated added to them if they do not already
        have them as dependencies.  Essentially the DAG is repaired to
        skip over the annihilated branch.

        Normally, this command will remove only an empty branch
        (base == head, except for changes to the .top* files); use
        ``-f`` to annihilate a non-empty branch.

        After completing the annihilation itself, normally ``tg update``
        is run on any modified dependents.  Use the ``--no-update`` option
        to suppress running ``tg update``.

tg depend
~~~~~~~~~
        Change the dependencies of a TopGit-controlled topic branch.
        This should have several subcommands, but only ``add`` is
        supported right now.

        The ``add`` subcommand takes an argument naming a topic branch to
        be added, adds it to ``.topdeps``, performs a commit and then
        updates your topic branch accordingly.  If you want to do other
        things related to the dependency addition, like adjusting
        ``.topmsg``, use the option ``--no-commit``.  Adding the
        ``--no-update`` (or ``--no-commit``) option will suppress the
        ``tg update`` normally performed after committing the change.

        It is safe to run ``tg depend add`` in a dirty worktree, but the
        normally performed ``tg update`` will be suppressed in that case
        (even if neither ``--no-update`` nor ``--no-commit`` is given).

        You have enabled ``git rerere`` haven't you?

        | TODO: Subcommand for removing dependencies, obviously

tg files
~~~~~~~~
        List files changed by the current or specified topic branch.

        Options:
          -i            list files based on index instead of branch
          -w            list files based on working tree instead of branch

tg info
~~~~~~~
        Show summary information about the current or specified topic
        branch.

        Numbers in parenthesis after a branch name such as "(11/3 commits)"
        indicate how many commits on the branch (11) and how many of those
        are non-merge commits (3).

        With ``--verbose`` (or ``-v``) include a list of dependents (i.e. other
        branches that depend on this one).  Another ``--verbose`` annotates
        them with "[needs merge]" if the current tip of branch for which info
        is being shown has not yet been merged into the base of the dependent.

        Alternatively, if ``--heads`` is used then which of the independent
        TopGit branch heads (as output by ``tg summary --topgit-heads``)
        logically contains the specified commit (which may be any committish --
        defaults to ``HEAD`` if not given).  Zero or more results will be
        output.  Note that "logically" means with regard to the TopGit
        dependency relationships as established by the ``.topdeps`` file(s).
        It's the answer that would be given when all the TopGit branches are
        up-to-date (even though they need not be to use this option) and the
        ``git branch --contains`` command is run and the output then filtered
        to only those branches that appear in ``tg summary --topgit-heads``.
        This computation may require several seconds on complex repositories.

        If ``--leaves`` is used then the unique list of leaves of the current
        or specified topic branch is shown as one fully-qualified ref per line.
        Duplicates are suppressed and a tag name will be used when appropriate.
        A "leaf" is any dependency that is either not a TopGit branch or is
        the base of a non-annihilated TopGit branch with no non-annihilated
        dependencies.

        A linearized patch series can only be automatically created for a
        TopGit topic branch (including its recursive dependencies) when exactly
        one line is output by ``tg info --leaves <topic-branch>``.

tg patch
~~~~~~~~
        Generate a patch from the current or specified topic branch.
        This means that the diff between the topic branch base and head
        (latest commit) is shown, appended to the description found in
        the ``.topmsg`` file.

        The patch is simply dumped to stdout.  In the future, ``tg patch``
        will be able to automatically send the patches by mail or save
        them to files. (TODO)

        Options:
          -i            base patch generation on index instead of branch
          -w            base patch generation on working tree instead of branch
          --binary      pass --binary to ``git diff-tree`` to enable generation
                        of binary patches
          --diff-opt    options after the branch name (and an optional ``--``)
                        are passed directly to ``git diff-tree``

        In order to pass a sole explicit ``-w`` through to ``git diff-tree`` it
        must be separated from the ``tg`` options by an explicit ``--``.
        Or it can be spelled as ``--ignore-all-space`` to distinguuish it from
        ``tg``'s ``-w`` option.

        If additional non-``tg`` options are passed through to
        ``git diff-tree`` (other than ``--binary`` which is fully supported)
        the resulting ``tg patch`` output may not be appliable.

tg mail
~~~~~~~
        Send a patch from the current or specified topic branch as
        email(s).

        Takes the patch given on the command line and emails it out.
        Destination addresses such as To, Cc and Bcc are taken from the
        patch header.

        Since it actually boils down to ``git send-email``, please refer
        to the documentation for that for details on how to setup email
        for git.  You can pass arbitrary options to this command through
        the ``-s`` parameter, but you must double-quote everything.  The
        ``-r`` parameter with a msgid can be used to generate in-reply-to
        and reference headers to an earlier mail.

        WARNING: be careful when using this command.  It easily sends
        out several mails.  You might want to run::

                git config sendemail.confirm always

        to let ``git send-email`` ask for confirmation before sending any
        mail.

        Options:
          -i            base patch generation on index instead of branch
          -w            base patch generation on working tree instead of branch

        | TODO: ``tg mail patchfile`` to mail an already exported patch
        | TODO: mailing patch series
        | TODO: specifying additional options and addresses on command line

tg remote
~~~~~~~~~
        Register the given remote as TopGit-controlled. This will create
        the namespace for the remote branch bases and teach ``git fetch``
        to operate on them. However, from TopGit 0.8 onwards you need to
        use ``tg push``, or ``git push --mirror``, for pushing
        TopGit-controlled branches.

        ``tg remote`` takes an optional remote name argument, and an
        optional ``--populate`` switch.  Use ``--populate`` for your
        origin-style remotes: it will seed the local topic branch system
        based on the remote topic branches.  ``--populate`` will also make
        ``tg remote`` automatically fetch the remote, and ``tg update`` look
        at branches of this remote for updates by default.

        Using ``--populate`` with a remote name causes the ``topgit.remote``
        git configuration variable to be set to the given remote name.

tg summary
~~~~~~~~~~
        Show overview of all TopGit-tracked topic branches and their
        up-to-date status.  With a branch name limit output to that branch.
        Using ``--deps-only`` or ``--rdeps`` changes the default from all
        branches to just the current ``HEAD`` branch but using ``--all`` as
        the branch name will show results for all branches instead of ``HEAD``.

                ``>``
                        marks the current topic branch

                ``0``
                        indicates that it introduces no changes of its own

                ``l``/``r``
                        indicates respectively whether it is local-only
                        or has a remote mate

                ``L``/``R``
                        indicates respectively if it is ahead or out-of-date
                        with respect to its remote mate

                ``D``
                        indicates that it is out-of-date with respect to its
                        dependencies

                ``!``
                        indicates that it has missing dependencies [even if
                        they are recursive ones]

                ``B``
                        indicates that it is out-of-date with respect to
                        its base

                ``*``
                        indicates it is ahead of (and needs to be merged into)
                        at least one of its dependents -- only computed when
                        showing all branches or using the (possibly implied)
                        ``--with-deps`` option.

        This can take a longish time to accurately determine all the
        relevant information about each branch; you can pass ``-t`` (or ``-l``
        or ``--list``) to get just a terse list of topic branch names quickly.

        Passing ``--heads`` shows independent topic branch names and when
        combined with ``--rdeps`` behaves as though ``--rdeps`` were run with
        the output of ``--heads``.

        The ``--heads-independent`` option works just like ``--heads`` except
        that it computes the heads using ``git merge-base --independent``
        rather than examining the TopGit ``.topdeps`` relationships.  If the
        TopGit branches are all up-to-date (as shown in ``tg summary``) then
        both ``--heads`` and ``--heads-independent`` should compute the same
        list of heads (unless some overlapping TopGit branches have been
        manually created).  If not all the TopGit branches are up-to-date then
        the ``--heads-independent`` results may have extra items in it, but
        occasionally that's what's needed; usually it's the wrong answer.
        (Note that ``--topgit-heads`` is accepted as an alias for ``--heads``
        as well.)

        Using ``--heads-only`` behaves as though the output of ``--heads`` was
        passed as the list of branches along with ``--without-deps``.

        Alternatively, you can pass ``--graphviz`` to get a dot-suitable output
        for drawing a dependency graph between the topic branches.

        You can also use the ``--sort`` option to sort the branches using
        a topological sort.  This is especially useful if each
        TopGit-tracked topic branch depends on a single parent branch,
        since it will then print the branches in the dependency order.
        In more complex scenarios, a text graph view would be much more
        useful, but that has not yet been implemented.

        The ``--deps`` option outputs dependency information between
        branches in a machine-readable format.  Feed this to ``tsort`` to
        get the output from --sort.

        The ``--deps-only`` option outputs a sorted list of the unique branch
        names given on the command line plus all of their recursive
        dependencies (subject to ``--exclude`` of course).  When
        ``--deps-only`` is given the default is to just display information for
        ``HEAD``, but that can be changed by using ``--all`` as the branch
        name.  Each branch name will appear only once in the output no matter
        how many times it's visited while tracing the dependency graph or how
        many branch names are given on the command line to process.

        The ``--rdeps`` option outputs dependency information in an indented
        text format that clearly shows all the dependencies and their
        relationships to one another.  When ``--rdeps`` is given the default is
        to just display information for ``HEAD``, but that can be changed by
        using ``--all`` as the branch name or by adding the ``--heads`` option.
        Note that ``tg summary --rdeps --heads`` can be particularly helpful in
        seeing all the TopGit-controlled branches in the repository and their
        relationships to one another.

        Adding ``--with-deps`` replaces the given list of branches (which will
        default to ``HEAD`` if none are given) with the result of running
        ``tg summary --deps-only --tgish`` on the list of branches.  This can
        be helpful in limiting ``tg summary`` output to only the list of given
        branches and their dependencies when many TopGit-controlled branches
        are present in the repository.  When it would be allowed,
        ``--with-deps`` is now the default.  Use ``--without-deps`` to switch
        back to the old behavior.

        With ``--exclude branch``, branch can be excluded from the output
        meaning it will be skipped and its name will be omitted from any
        dependency output.  The ``--exclude`` option may be repeated to omit
        more than one branch from the output.  Limiting the output to a single
        branch that has been excluded will result in no output at all.

        The ``--tgish-only`` option behaves as though any non-TopGit-controlled
        dependencies encountered during processing had been listed after an
        ``--exclude`` option.

        Note that the branch name can be specified as ``HEAD`` or ``@`` as a
        shortcut for the TopGit-controlled branch that ``HEAD`` is a
        symbolic ref to.  The ``tg summary @`` command can be quite useful.

        Options:
          -i            Use TopGit metadata from the index instead of the branch
          -w            Use TopGit metadata from the working tree instead of the branch

tg contains
~~~~~~~~~~~
        Search all TopGit-controlled branches (and optionally their remotes)
        to find which TopGit-controlled branch contains the specified commit.

        This is more than just basic branch containment as provided for by the
        ``git branch --contains`` command.  While the shown branch name(s)
        will, indeed, be one (or more) of those output by the
        ``git branch --contains`` command, the result(s) will exclude any
        TopGit-controlled branches from the result(s) that have one (or more)
        of their TopGit dependencies (either direct or indirect) appearing in
        the ``git branch --contains`` output.

        Normally the result will be only the one, single TopGit-controlled
        branch for which the specified committish appears in the ``tg log``
        output for that branch (unless the committish lies outside the
        TopGit-controlled portion of the DAG and ``--no-strict`` was used).

        Unless ``--annihilated-okay`` (or ``--ann`` or ``--annihilated``) is
        used then annihilated branches will be immediately removed from the
        ``git branch --contains`` output before doing anything else.  This
        means a committish that was originally located in a now-annihilated
        branch will show up in whatever branch picked up the annihilated
        branch's changes (if there is one).  This is usually the correct
        answer, but occasionally it's not; hence this option.  If this option
        is used together with ``--verbose`` then annihilated branches will
        be shown as "[:annihilated:]".

        In other words, if a ``tg patch`` is generated for the found branch
        (assuming one was found and a subsequent commit in the same branch
        didn't then revert or otherwise back out the change), then that patch
        will include the changes introduced by the specified committish
        (unless, of course, that committish is outside the TopGit-controlled
        portion of the DAG and ``--no-strict`` was given).

        This can be very helpful when, for example, a bug is discovered and
        then after using ``git bisect`` (or some other tool) to find the
        offending commit it's time to commit the fix.  But because the
        TopGit merging history can be quite complicated and maybe the one
        doing the fix wasn't the bug's author (or the author's memory is just
        going), it can sometimes be rather tedious to figure out which
        TopGit branch the fix belongs in.  The ``tg contains`` command can
        quickly tell you the answer to that question.

        With the ``--remotes`` (or ``-r``) option a TopGit-controlled remote
        branch name may be reported as the result but only if there is no
        non-remote branch containing the committish (this can only happen
        if at least one of the TopGit-controlled local branches are not yet
        up-to-date with their remotes).

        With the ``--verbose`` option show which TopGit DAG head(s) (one or
        more of the TopGit-controlled branch names output by
        ``tg summary --heads``) have the result as a dependency (either direct
        or indirect).  Using this option will noticeably increase running time.

        With the default ``--strict`` option, results for which the base of the
        TopGit-controlled branch contains the committish will be suppressed.
        For example, if the committish was deep-down in the master branch
        history somewhere far outside of the TopGit-controlled portion of
        the DAG, with ``--no-strict``, whatever TopGit-controlled branch(es)
        first picked up history containing that committish will be shown.
        While this is a useful result it's usually not the desired result
        which is why it's not the default.

        To summarize, even with ``--remotes``, remote results are only shown
        if there are no non-remote results.  Without ``--no-strict`` (because
        ``--strict`` is the default) results outside the TopGit-controlled
        portion of the DAG are never shown and even with ``--no-strict`` they
        will only be shown if there are no ``--strict`` results.  Finally,
        the TopGit head info shown with ``--verbose`` only ever appears for
        local (i.e. not a remote branch) results.  Annihilated branches are
        never considered possible matches without ``--annihilated-okay``.

tg checkout
~~~~~~~~~~~
        Switch to a topic branch.  You can use ``git checkout <branch>``
        to get the same effect, but this command helps you navigate
        the dependency graph, or allows you to match the topic branch
        name using a regular expression, so it can be more convenient.

        There following subcommands are available:

            ``tg checkout push``
                                Check out a branch that directly
                                depends on your current branch.

            ``tg checkout pop``
                                Check out a branch that this branch
                                directly depends on.

            ``tg checkout [goto] [--] <pattern>``
                                Check out a topic branch that
                                matches ``<pattern>``.  ``<pattern>``
                                is used as a sed pattern to filter
                                all the topic branches.  Both ``goto`` and
                                ``--`` may be omitted provided ``<pattern>``
                                is not ``push``, ``pop``, ``-a``, ``--all``,
                                ``goto``, ``..``, ``--``, ``next``, ``child``,
                                ``prev``, ``parent``, ``-h`` or ``--help``.

            ``tg checkout next``
                                An alias for ``push``.

            ``tg checkout child``
                                An alias for ``push``.

            ``tg checkout``
                                An alias for ``push``.

            ``tg checkout prev``
                                An alias for ``pop``.

            ``tg checkout parent``
                                An alias for ``pop``.

            ``tg checkout ..``
                                An alias for ``pop``.

        If any of the above commands can find more than one possible
        branch to switch to, you will be presented with the matches
        and asked to select one of them.

        If the ``--ignore-other-worktrees`` (or ``--iow``) option is given and
        the current Git version is at least 2.5.0 then the full
        ``--ignore-other-worktrees`` option will be passed along to the
        ``git checkout`` command when it's run (otherwise the option will be
        silently ignored and not passed to Git as it would cause an error).

        The ``--force`` (or ``-f``) option, when given, gets passed through to
        the ``git checkout`` command.

        The ``<pattern>`` of ``tg checkout goto`` is optional.  If you don't
        supply it, all the available topic branches are listed and you
        can select one of them.

        Normally, the ``push`` and ``pop`` commands moves one step in
        the dependency graph of the topic branches.  The ``-a`` option
        causes them (and their aliases) to move as far as possible.
        That is, ``tg checkout push -a`` moves to a topic branch that
        depends (directly or indirectly) on the current branch and
        that no other branch depends on.  ``tg checkout pop -a``
        moves to a regular branch that the current topic branch
        depends on (directly or indirectly).  If there is more than
        one possibility, you will be prompted for your selection.

tg export
~~~~~~~~~
        Export a tidied-up history of the current topic branch and its
        dependencies, suitable for feeding upstream.  Each topic branch
        corresponds to a single commit or patch in the cleaned up
        history (corresponding basically exactly to ``tg patch`` output
        for the topic branch).

        The command has three possible outputs now -- either a Git branch
        with the collapsed history, a Git branch with a linearized
        history, or a quilt series in new directory.

        In the case where you are producing collapsed history in a new
        branch, you can use this collapsed structure either for
        providing a pull source for upstream, or for further
        linearization e.g. for creation of a quilt series using git log::

                git log --pretty=email -p --topo-order origin..exported

        To better understand the function of ``tg export``, consider this
        dependency structure::

         origin/master - t/foo/blue - t/foo/red - master
                      `- t/bar/good <,----------'
                      `- t/baz      ------------'

        (where each of the branches may have a hefty history). Then::

         master$ tg export for-linus

        will create this commit structure on the branch ``for-linus``::

         origin/master - t/foo/blue -. merge - t/foo/red -.. merge - master
                      `- t/bar/good <,-------------------'/
                      `- t/baz      ---------------------'

        In this mode, ``tg export`` works on the current topic branch, and
        can be called either without an option (in that case,
        ``--collapse`` is assumed), or with the ``--collapse`` option, and
        with one mandatory argument: the name of the branch where the
        exported result will be stored.

        When using the linearize mode::

         master$ tg export --linearize for-linus

        you get a linear history respecting the dependencies of your
        patches in a new branch ``for-linus``.  The result should be more
        or less the same as using quilt mode and then reimporting it
        into a Git branch.  (More or less because the topological order
        can usually be extended in more than one way into a total order,
        and the two methods may choose different ones.)  The result
        might be more appropriate for merging upstream, as it contains
        fewer merges.

        Note that you might get conflicts during linearization because
        the patches are reordered to get a linear history.  If linearization
        would produce conflicts then using ``--quilt`` will also likely result
        in conflicts when the exported quilt series is applied.  Since the
        ``--quilt`` mode simply runs a series of ``tg patch`` commands to
        generate the patches in the exported quilt series and those patches
        will end up being applied linearly, the same conflicts that would be
        produced by the ``--linearize`` option will then occur at that time.

        To avoid conflicts produced by ``--linearize`` (or by applying the
        ``--quilt`` output), use the default ``--collapse`` mode and then use
        ``tg rebase`` (or ``git rebase -m`` directly) on the collapsed branch
        (with a suitable <upstream>) followed by ``git format-patch`` on the
        rebased result to produce a conflict-free patch set.  A suitable
        upstream may be determined with the ``tg info --leaves`` command (if
        it outputs more than one line, linearization will be problematic).

        You have enabled ``git rerere`` haven't you?

        When using the quilt mode::

         master$ tg export --quilt for-linus

        would create the following directory ``for-linus``::

         for-linus/t/foo/blue.diff
         for-linus/t/foo/red.diff
         for-linus/t/bar/good.diff
         for-linus/t/baz.diff
         for-linus/series:
                t/foo/blue.diff -p1
                t/bar/good.diff -p1
                t/foo/red.diff -p1
                t/baz.diff -p1

        With ``--quilt``, you can also pass the ``-b`` parameter followed
        by a comma-separated explicit list of branches to export, or
        the ``--all`` parameter (which can be shortened to ``-a``) to
        export them all.  The ``--binary`` option enables producing Git
        binary patches.  These options are currently only supported
        with ``--quilt``.

        In ``--quilt`` mode the patches are named like the originating
        topgit branch.  So usually they end up in subdirectories of the
        output directory.  With the ``--flatten`` option the names are
        mangled so that they end up directly in the output dir (slashes
        are replaced with underscores).  With the ``--strip[=N]`` option
        the first ``N`` subdirectories (all if no ``N`` is given) get
        stripped off.  Names are always ``--strip``'d before being
        ``--flatten``'d.  With the option ``--numbered`` (which implies
        ``--flatten``) the patch names get a number as prefix to allow
        getting the order without consulting the series file, which
        eases sending out the patches.

        | TODO: Make stripping of non-essential headers configurable
        | TODO: Make stripping of [PATCH] and other prefixes configurable
        | TODO: ``--mbox`` option to export instead as an mbox file
        | TODO: support ``--all`` option in other modes of operation
        | TODO: For quilt exporting, export the linearized history created in
                a temporary branch--this would allow producing conflict-less
                series

tg import
~~~~~~~~~
        Import commits within the given revision range into TopGit,
        creating one topic branch per commit. The dependencies are set
        up to form a linear sequence starting on your current branch --
        or a branch specified by the ``-d`` parameter, if present.

        The branch names are auto-guessed from the commit messages and
        prefixed by ``t/`` by default; use ``-p <prefix>`` to specify an
        alternative prefix (even an empty one).

        Alternatively, you can use the ``-s NAME`` parameter to specify
        the name of the target branch; the command will then take one
        more argument describing a *single* commit to import.

tg update
~~~~~~~~~
        Update the current, specified or all topic branches with respect
        to changes in the branches they depend on and remote branches.
        This is performed in two phases -- first, changes within the
        dependencies are merged to the base, then the base is merged
        into the topic branch.  The output will guide you on what to do
        next in case of conflicts.

        You have enabled ``git rerere`` haven't you?

        When ``-a`` (or ``--all``) is specifed, updates all topic branches
        matched by ``<pattern>``'s (see ``git-for-each-ref(1)`` for details),
        or all if no ``<pattern>`` is given.  Any topic branches with missing
        dependencies will be skipped entirely unless ``--skip-missing`` is
        specified.

        When ``--skip-missing`` is specifed, an attempt is made to update topic
        branches with missing dependencies by skipping only the dependencies
        that are missing.  Caveat utilitor.

        When ``--stash`` is specified (or the ``topgit.autostash`` config
        value is set to ``true``), a ref stash will be automatically created
        just before beginning updates if any are needed.  The ``--no-stash``
        option may be used to disable a ``topgit.autostash=true`` setting.
        See the ``tg tag`` ``--stash`` option for details.

        After the update, if a single topic branch was specified, it is
        left as the current one; if ``-a`` was specified, it returns to
        the branch which was current at the beginning.

        If your dependencies are not up-to-date, ``tg update`` will first
        recurse into them and update them.

        If a remote branch update brings in dependencies on branches
        that are not yet instantiated locally, you can either bring in
        all the new branches from the remote using ``tg remote
        --populate``, or only pick out the missing ones using ``tg create
        -r`` (``tg summary`` will point out branches with incomplete
        dependencies by showing an ``!`` next to them).

        Using the alternative ``--base`` mode, ``tg update`` will update
        the base of a specified ``[BASE]`` branch (which is a branch created
        by ``tg create`` using the ``--base`` option) to the specified
        committish (the second argument) and then immediately merge that into
        the branch itself using the specified message for the merge commit.
        If no message is specified on the command line, an editor will open.
        Unless ``--force`` is used the new value for the base must contain
        the old value (i.e. be a fast-forward update).  This is for safety.

        This mode makes updates to ``[BASE]`` branches quick and easy.

        | TODO: ``tg update -a -c`` to autoremove (clean) up-to-date branches

tg push
~~~~~~~
        If ``-a`` or ``--all`` was specified, pushes all non-annihilated
        TopGit-controlled topic branches, to a remote repository.
        Otherwise, pushes the specified topic branches -- or the
        current branch, if you don't specify which.  By default, the
        remote gets all the dependencies (both TopGit-controlled and
        non-TopGit-controlled) and bases pushed to it too.  If
        ``--tgish-only`` was specified, only TopGit-controlled
        dependencies will be pushed, and if ``--no-deps`` was specified,
        no dependencies at all will be pushed.

        The ``--dry-run`` and ``--force`` options are passed directly to
        ``git push`` if given.

        The remote may be specified with the ``-r`` option. If no remote
        was specified, the configured default TopGit remote will be
        used.

tg base
~~~~~~~
        Prints the base commit of each of the named topic branches, or
        the current branch if no branches are named.  Prints an error
        message and exits with exit code 1 if the named branch is not
        a TopGit branch.

tg log
~~~~~~
        Prints the git log of the named topgit branch -- or the current
        branch, if you don't specify a name.

        This is really just a convenient shortcut for:

            ``git log --first-parent --no-merges $(tg base <name>)..<name>``

        where ``<name>`` is the name of the TopGit topic branch (or omitted
        for the current branch).

        However, if ``<name>`` is a ``[BASE]`` branch the ``--no-merges``
        option is omitted.

        If ``--compact`` is used then ``git log-compact`` will be used instead
        of ``git log``.  The ``--command=<git-alias>`` option can be used to
        replace "log" with any non-whitespace-containing command alias name,
        ``--compact`` is just a shortcut for ``--command=log-compact``.  The
        ``git-log-compact`` tool may be found on its project page located at:

            https://mackyle.github.io/git-log-compact

        Note that the ``--compact`` or ``--command=`` option must be used
        before any ``--`` or ``git log`` options to be recognized.

        NOTE: if you have merged changes from a different repository, this
        command might not list all interesting commits.

tg tag
~~~~~~
        Creates a TopGit annotated/signed tag or lists the reflog of one.

        A TopGit annotated tag records the current state of one or more TopGit
        branches and their dependencies and may be used to revert to the tagged
        state at any point in the future.

        When reflogs are enabled (the default in a non-bare repository) and
        combined with the ``--force`` option a single tag name may be used as a
        sort of TopGit branch state stash.  The special branch name ``--all``
        may be used to tag the state of all current TopGit branches to
        facilitate this function and has the side-effect of suppressing the
        out-of-date check allowing out-of-date branches to be included.

        As a special feature, ``--stash`` may be used as the tag name in which
        case ``--all`` is implied if no branch name is listed (instead of the
        normal default of ``HEAD``), ``--force`` and ``--no-edit`` (use
        ``--edit`` to change that) are automatically activated and the tag will
        be saved to ``refs/tgstash`` instead of ``refs/tags/<tagname>``.
        The ``--stash`` tag name may also be used with the ``-g``/``--reflog``
        option.

        The mostly undocumented option ``--allow-outdated`` will bypass the
        out-of-date check and is implied when ``--stash`` or ``--all`` is used.

        A TopGit annotated/signed tag is simply a Git annotated/signed tag with
        a "TOPGIT REFS" section appended to the end of the tag message (and
        preceding the signature for signed tags).  PEM-style begin and end
        lines surround one line per ref where the format of each line is
        full-hash SP ref-name.  A line will be included for each branch given
        on the command line and each ref they depend on either directly or
        indirectly.

        If more than one TopGit branch is given on the command line, a new
        commit will be created that has an empty tree and all of the given
        TopGit branches as parents and that commit will be tagged.  If a single
        TopGit branch is given, then it will be tagged.  If the ``--tree``
        option is used then it will be used instead of an empty tree (a new
        commit will be created if necessary to guarantee the specified tree is
        what's in the commit the newly created tag refers to).  The argument to
        the ``--tree`` option may be any valid treeish.

        All the options for creating a tag serve the same purpose as their Git
        equivalents except for two.  The ``--refs`` option suppresses tag
        creation entirely and emits the "TOPGIT REFS" section that would have
        been included with the tag.  If the ``--no-edit`` option is given and
        no message is supplied (via the ``-m`` or ``-F`` option) then the
        default message created by TopGit will be used without running the
        editor.

        With ``-g`` or ``--reflog`` show the reflog for a tag.  With the
        ``--reflog-message`` option the message from the reflog is shown.
        With the ``--commit-message`` option the first line of the tag's
        message (if the object is a tag) or the commit message (if the object
        is a commit) falling back to the reflog message for tree and blob
        objects is shown.  The default is ``--reflog-message`` unless the
        ``--stash`` (``refs/tgstash``) is being shown in which case the default
        is then ``--commit-message``.  Just add either option explicitly to
        override the default.

        When showing reflogs, non-tag entries are annotated with their type
        unless ``--no-type`` is given.

        TopGit tags are created with a reflog if core.logallrefupdates is
        enabled (the default for non-bare repositories).  Unfortunately Git
        is incapable of showing an annotated/signed tag's reflog
        (using git log -g) as it will first resolve the tag before checking to
        see if it has a reflog.  Git can, however, show reflogs for lightweight
        tags (using git log -g) just fine but that's not helpful here.  Use
        ``tg tag`` with the ``-g`` or ``--reflog`` option to see the reflog for
        an actual tag object.  This also works on non-TopGit annotated/signed
        tags as well provided they have a reflog.

        The number of entries shown may be limited with the ``-n`` option.  If
        the tagname is omitted then ``--stash`` is assumed.

        The ``--delete`` option is a convenience option that runs the
        ``git update-ref -d`` command on the specified tag removing it and its
        reflog (if it has one).

        The ``--clear`` option clears all but the most recent (the ``@{0}``)
        reflog entries from the reflog for the specified tag.  It's equivalent
        to dropping all the higher numbered reflog entries.

        The ``--drop`` option drops the specified reflog entry and requires the
        given tagname to have an ``@{n}`` suffix where ``n`` is the reflog
        entry number to be dropped.   This is really just a convenience option
        that runs the appropriate ``git reflog delete`` command.

        Note that when combined with ``tg revert``, a tag created by ``tg tag``
        can be used to transfer TopGit branches.  Simply create the tag, push
        it somewhere and then have the recipient run ``tg revert`` to recreate
        the TopGit branches.  This may be helpful in situations where it's not
        feasible to push all the refs corresponding to the TopGit-controlled
        branches and their top-bases.

tg rebase
~~~~~~~~~
        Provides a ``git rebase`` rerere auto continue function.  It may be
        used as a drop-in replacement front-end for ``git rebase -m`` that
        automatically continues the rebase when ``git rerere`` information is
        sufficient to resolve all conflicts.

        You have enabled ``git rerere`` haven't you?

        If the ``-m`` or ``--merge`` option is not present then ``tg rebase``
        will complain and not do anything.

        When ``git rerere`` is enabled, previously resolved conflicts are
        remembered and can be automatically staged (see ``rerere.autoUpdate``).

        However, even with auto staging, ``git rebase`` still stops and
        requires an explicit ``git rebase --continue`` to keep going.

        In the case where ``git rebase -m`` is being used to flatten history
        (such as after a ``tg export --collapse`` prior to a
        ``git format-patch``), there's a good chance all conflicts have already
        been resolved during normal merge maintenance operations so there's no
        reason ``git rebase`` could not automatically continue, but there's no
        option to make it do so.

        The ``tg rebase`` command provides a ``git rebase --auto-continue``
        function.

        All the same rebase options can be used (they are simply passed through
        to Git unchanged).  However, the ``rerere.autoUpdate`` option is
        automatically temporarily enabled while running ``git rebase`` and
        should ``git rebase`` stop asking one to resolve and continue, but all
        conflicts have already been resolved and staged using rerere
        information, then ``git rebase --continue`` will be automatically run.

tg revert
~~~~~~~~~
        Provides the ability to revert one or more TopGit branches and their
        dependencies to a previous state contained within a tag created using
        the ``tg tag`` command.  In addition to the actual revert mode
        operation a list mode operation is also provided to examine a tag's ref
        contents.

        The default mode (``-l`` or ``--list``) shows the state of one or more
        of the refs/branches stored in the tag data.  When no refs are given on
        the command line, all refs in the tag data are shown.  With the special
        ref name ``--heads`` then the indepedent heads contained in the tag
        data are shown.  The ``--deps`` option shows the specified refs and all
        of their dependencies in a single list with no duplicates.  The
        ``--rdeps`` option shows a display similar to ``tg summary --rdeps``
        for each ref or all TopGit heads if no ref is given on the command
        line.  The standard ``--no-short``, ``--short=n`` etc. options may be
        used to override the default ``--short`` output.  With ``--hash`` (or
        ``--hash-only``) show only the hash in ``--list`` mode in which case
        the default is ``--no-short``.   The ``--hash`` option can be used much
        like the ``git rev-parse --verify`` command to extract a specific hash
        value out of a TopGit tag.

        Note that unlike `tg summary`_, here ``--heads`` actually does mean the
        ``git merge-base --independent`` heads of the stored refs from the tag
        data.  To see only the independent TopGit topic branch heads stored in
        the tag data use the ``--topgit-heads`` option instead.  The default
        for the ``--rdeps`` option is ``--topgit-heads`` but ``--heads`` can
        be given explicitly to change that.  (Note that ``--heads-independent``
        is accepted as an alias for ``--heads`` as well.)

        The revert mode has three submodes, dry-run mode (``-n`` or
        ``--dry-run``), force mode (``-f`` or ``--force``) and interactive mode
        (``-i`` or ``--interactive``).  If ``--dry-run`` (or ``-n``) is given
        no ref updates will actually be performed but what would have been
        updated is shown instead.  If ``--interactive`` (or ``-i``) is given
        then the editor is invoked on an instruction sheet allowing manual
        selection of the refs to be updated before proceeding.  Since revert is
        potentially a destructive operation, at least one of the submodes must
        be specified explicitly.  If no refs are listed on the command line
        then all refs in the tag data are reverted.  Otherwise the listed refs
        and all of their dependencies (unless ``--no-deps`` is given) are
        reverted.  Unless ``--no-stash`` is given a new stash will be created
        using ``tg tag --stash`` (except, of course, in dry-run mode) just
        before actually performing the updates to facilitate recovery from
        accidents.

        Both modes accept fully-qualified (i.e. starts with ``refs/``) ref
        names as well as unqualified names (which will be assumed to be located
        under ``refs/heads/``).  In revert mode a tgish ref will always have
        both its ``refs/heads/`` and ``refs/top-bases/`` values included no
        matter how it's listed unless ``--no-deps`` is given and the ref is
        fully qualified (i.e. starts with ``refs/``) or one or the other of its
        values was removed from the instruction sheet in interactive mode.  In
        list mode a tgish ref will always have both its ``refs/heads/`` and
        ``refs/top-bases/`` values included only when using the ``--deps`` or
        ``--rdeps`` options.

        The ``--tgish-only`` option excludes non-tgish refs (i.e. refs that do
        not have a ``refs/heads/<name>``, ``refs/top-bases/<name>`` pair).

        The ``--exclude`` option (which can be repeated) excludes specific
        refs.  If the name given to ``--exclude`` is not fully-qualified (i.e.
        starts with ``refs/``) then it will exclude both members of a tgish ref
        pair.

        The ``--quiet`` (or ``-q``) option may be used in revert mode to
        suppress non-dry-run ref change status messages.

        The special tag name ``--stash`` (as well as with ``@{n}`` suffixes)
        can be used to refer to ``refs/tgstash``.

        The ``tg revert`` command supports tags of tags that contains TopGit
        refs.  So, for example, if you do this::

                tg tag newtag --all
                git tag -f -a -m "tag the tag" newtag newtag

        Then ``newtag`` will be a tag of a tag containing a ``TOPGIT REFS``
        section.  ``tg revert`` knows how to dereference the outermost
        tag to get to the next (and the next etc.) tag to find the
        ``TOPGIT REFS`` section so after the above sequence, the tag ``newtag``
        can still be used successfully with ``tg revert``.

        NOTE:  If HEAD points to a ref that is updated by a revert operation
        then NO WARNING whatsoever will be issued, but the index and working
        tree will always be left completely untouched (and the reflog for
        the pointed-to ref can always be used to find the previous value).

tg prev
~~~~~~~
        Outputs the direct dependencies for the current or named branch.

        Options:
          -i            show dependencies based on index instead of branch
          -w            show dependencies based on working tree instead of branch

tg next
~~~~~~~
        Outputs all branches which directly depend on the current or
        named branch.

        Options:
          -i            show dependencies based on index instead of branch
          -w            show dependencies based on working tree instead of branch

tg migrate-bases
~~~~~~~~~~~~~~~~
        Transition top-bases from old location to new location.

        Beginning with TopGit release 0.19.4, TopGit has the ability to store
        the top-bases refs in either the old ``ref/top-bases/...`` location or
        the new ``refs/heads/{top-bases}/...`` location.  Starting with TopGit
        release 0.20.0, the default is the new location.

        By storing the top-bases under heads, Git is less likely to complain
        when manipulating them, hosting providers are more likely to provide
        access to them and Git prevents them from pointing at anything other
        than a commit object.  All in all a win for everyone.

        TopGit attempts to automatically detect whether the new or old location
        is being used for the top-bases and just do the right thing.  However,
        by explicitly setting the config value ``topgit.top-bases`` to either
        ``refs`` for the old location or ``heads`` for the new location the
        auto-detection can be bypassed.  If no top-bases refs are present in
        the repository the default prior to TopGit release 0.20.0 is to use the
        old location but starting with TopGit release 0.20.0 the default is to
        use the new location.

        The ``tg migrate-bases`` command may be used to migrate top-bases refs
        from the old location to the new location (or, by using the
        undocumented ``--reverse`` option, vice versa).

        With few exceptions (``tg create -r`` and ``tg revert``), all top-bases
        refs (both local *and* remote refs) are expected to be stored in the
        same location (either new or old).  A repository's current location for
        storing top-bases refs may be shown with the ``tg --top-bases`` command.

TODO: tg rename
~~~~~~~~~~~~~~~

IMPLEMENTATION
--------------

TopGit stores all the topic branches in the regular ``refs/heads/``
namespace (so we recommend distinguishing them with the ``t/`` prefix).
Apart from that, TopGit also maintains a set of auxiliary refs in
``refs/top-*``.  Currently, only ``refs/top-bases/`` is used, containing the
current *base* of the given topic branch -- this is basically a merge of
all the branches the topic branch depends on; it is updated during ``tg
update`` and then merged to the topic branch, and it is the base of a
patch generated from the topic branch by ``tg patch``.

All the metadata is tracked within the source tree and history of the
topic branch itself, in ``.top*`` files; these files are kept isolated
within the topic branches during TopGit-controlled merges and are of
course omitted during ``tg patch``.  The state of these files in base
commits is undefined; look at them only in the topic branches
themselves.  Currently, two files are defined:

        ``.topmsg``:
            Contains the description of the topic branch in a
            mail-like format, plus the author information, whatever
            Cc headers you choose or the post-three-dashes message.
            When mailing out your patch, basically only a few extra
            mail headers are inserted and then the patch itself is
            appended.  Thus, as your patches evolve, you can record
            nuances like whether the particular patch should have
            To-list / Cc-maintainer or vice-versa and similar
            nuances, if your project is into that.  ``From`` is
            prefilled from your current ``GIT_AUTHOR_IDENT``; other
            headers can be prefilled from various optional
            ``topgit.*`` git config options.

        ``.topdeps``:
            Contains the one-per-line list of branches this branch
            depends on, pre-seeded by ``tg create``. A (continuously
            updated) merge of these branches will be the *base* of
            your topic branch.

IMPORTANT: DO NOT EDIT ``.topdeps`` MANUALLY!!! If you do so, you need to
know exactly what you are doing, since this file must stay in sync with
the Git history information, otherwise very bad things will happen.

TopGit also automagically installs a bunch of custom commit-related
hooks that will verify whether you are committing the ``.top*`` files in a
sane state. It will add the hooks to separate files within the ``hooks/``
subdirectory, and merely insert calls to them to the appropriate hooks
and make them executable (but will make sure the original hook's code is
not called if the hook was not executable beforehand).

Another automagically installed piece is a ``.git/info/attributes``
specifier for an ``ours`` merge strategy for the files ``.topmsg`` and
``.topdeps``, and the (intuitive) ``ours`` merge strategy definition in
``.git/config``.


REMOTE HANDLING
---------------

There are two remaining issues with accessing topic branches in remote
repositories:

        (i) Referring to remote topic branches from your local repository
        (ii) Developing some of the remote topic branches locally

There are two somewhat contradictory design considerations here:

        (a) Hacking on multiple independent TopGit remotes in a single
            repository
        (b) Having a self-contained topic system in local refs space

To us, (a) does not appear to be very convincing, while (b) is quite
desirable for ``git-log topic`` etc. working, and increased conceptual
simplicity.

Thus, we choose to instantiate all the topic branches of given remote
locally; this is performed by ``tg remote --populate``. ``tg update``
will also check if a branch can be updated from its corresponding remote
branch.  The logic needs to be somewhat involved if we are to "do the
right thing".  First, we update the base, handling the remote branch as
if it was the first dependency; thus, conflict resolutions made in the
remote branch will be carried over to our local base automagically.
Then, the base is merged into the remote branch and the result is merged
to the local branch -- again, to carry over remote conflict resolutions.
In the future, this order might be adjustable on a per-update basis, in
case local changes happen to be diverging more than the remote ones.
(See the details in `The Update Process`_ for more in depth coverage.)

All commands by default refer to the remote that ``tg remote --populate``
was called on the last time (stored in the ``topgit.remote`` git
configuration variable). You can manually run any command with a
different base remote by passing ``-r REMOTE`` *before* the subcommand
name or passing ``-u`` *before* the subcommand to run without one.


TECHNICAL
---------

A familiarity with the terms in the GLOSSARY_ is helpful for understanding the
content of this sections.  See also the IMPLEMENTATION_ section.

The Update Process
~~~~~~~~~~~~~~~~~~

When a branch is "updated" using the ``tg update`` command the following steps
are taken:

        1) The branch and all of its dependencies (and theirs recursively)
           are checked to see which ones are *out-of-date*.  See glossary_.

        2) Each of the branch's direct dependencies (i.e. they are listed in
           the branch's ``.topdeps`` file) which is out of date is updated
           before proceeding (yup, this is a recursive process).

        3) Each of the branch's direct dependencies (i.e. they are listed in
           the branch's ``.topdes`` file) that was updated in the previous
           step is now merged into the branch's corresponding base.  If a
           remote is involved, and the branch's corresponding base does NOT
           contain the remote branch's corresponding base that remote base is
           also merged into the branch's base at this time as well (it will be
           the first item merged into the branch's base).

        4) If the branch has a corresponding remote branch and the branch
           does not already contain it, it's merged into the branch's base
           (which was possibly already updated in step (3) to contain the
           remote branch's base but not the remote branch itself) on a detached
           HEAD.  Yup, this step can be a bit confusing and no, the updated
           base from step (3) has not yet been merged into the branch itself
           yet either.  If there is no remote branch this step does not apply.
           Using a detached HEAD allows the remote branch to be merged into the
           contents of the base without actually perturbing the base's ref.

        5) If there is a remote branch present then use the result of step (4)
           otherwise use the branch's base and merge that into the branch
           itself.

That's it!  Simple, right? ;)

Unless the auto stash option has been disabled (see `no undo`_, `tg update`_
and `tg tag`_), a copy of all the old refs values will be stashed away
immediately after step (1) before starting step (2), but only if anything is
actually found to be out-of-date.

Merge Strategies
~~~~~~~~~~~~~~~~

The ``tg update`` command regularly performs merges while executing an update
operation.  In order to speed things up, it attempts to do in-index merges
where possible.  It accomplishes this by using a separate, temporary index
file and the ``git read-tree -m --aggressive`` command possibly assisted by
the ``git merge-index`` and ``git merge-file`` commands.  This combination may
be repeated more than once to perform an octopus in-index merge.  If this
fails, the files are checked out and a normal ``git merge`` three-way merge is
performed (possily multiple times).  If the normal ``git merge`` fails then
user intervention is required to resolve the merge conflict(s) and continue.

Since the ``tg annihilate``, ``tg create`` and ``tg depend add`` commands may
end up running the ``tg update`` machinery behind the scenes to complete their
operation they may also result in any of these merge strategies being used.

In addition to the normal Git merge strategies (if the in-index merging fails),
there are four possible TopGit merge strategies that may be shown.  Since they
all involve use of the ``git read-tree -m --aggressive`` command they are all
variations of a "trivial aggressive" merge.  The "trivial" part because all of
the merges done by ``git read-tree -m`` are described as "trivial" and the
"aggressive" part because the ``--aggressive`` option is always used.

        1) "trivial aggressive"
                Only two heads were involved and all merging was completed by
                the ``git read-tree -m --aggressive`` command.

        2) "trivial aggressive automatic"
                Only two heads were involved but after the
                ``git read-tree -m --aggressive`` command completed there were
                still unresolved items and ``git merge-index`` had to be run
                (using the ``tg index-merge-one-file`` driver) which ultimately
                ran ``git merge-file`` at least once to perform a simple
                automatic three-way merge.  Hence the "automatic" description
                and the "Auto-merging ..." output line(s).

        3) "trivial aggressive octopus"
                This is the same as a "trivial aggressive" merge except that
                more than two heads were involved and after merging the first
                two heads, the ``git read-tree -m --aggressive`` step was
                repeated again on the result for each additional head.  All
                merging was completed via multiple
                ``git read-tree -m --aggressive`` commands only.
                This beast is relatively rare in the wild.

        4) "trivial aggressive automatic octopus"
                This is very similar to the "trivial aggressive octopus"
                except that at least one of the ``git read-tree -m --aggressive``
                commands left unresolved items that were handled the same way
                as the "trivial aggressive automatic" strategy.  This species
                is commonly seen in the wild.


GLOSSARY
--------

        .topmsg
                Version-controlled file stored at the root level of each
                TopGit branch that contains the patch header for a TopGit
                branch.  See also IMPLEMENTATION_;

        .topdeps
                Version-controlled file stored at the root level of each
                TopGit branch that lists the branch's dependencies one per
                line omitting the leading ``refs/heads/`` part.  See also
                IMPLEMENTATION_;

        branch containment
                Given two Git commit identifiers (e.g. hashes) C1 and C2,
                commit C1 "contains" commit C2 if either they are the same
                commit or C2 can be reached from C1 by following one or more
                parent links from C1 (perhaps via one or more intermediate
                commits along the way).  In other words, if C1 contains C2
                then C2 is an ancestor of C1 or conversely C1 is a descendant
                of C2.  Since a TopGit branch name is also the name of a Git
                branch (something located under the ``refs/heads`` Git
                namespace) and similarly for a TopGit base, they can both be
                resolved to a Git commit identifier and then participate in
                a branch containment test.  An easy mnemonic for this is
                "children contain the genes of their parents."

        contains
                See branch containment.

        TopGit
                Excellent system for managing a history of changes to one
                or more possibly interrelated patches.

        TopGit branch
                A Git branch that has an associated TopGit base.  Conceptually
                it represents a single patch that is the difference between
                the associated TopGit base and the TopGit branch.  In other
                words ``git diff-tree <TopGit base> <TopGit branch>`` except
                that any ``.topdeps`` and/or ``.topmsg`` files are excluded
                from the result and the contents of the ``.topmsg`` file from
                the TopGit branch is prefixed to the result.

        TopGit base
                A Git branch that records the base upon which a TopGit branch's
                single conceptual "patch" is built.  The name of the Git branch
                is derived from the TopGit branch name by stripping off the
                leading ``refs/heads/`` and appending the correct prefix where
                all TopGit bases are stored (typically either
                ``refs/top-bases/`` or ``refs/heads/{top-bases}/`` -- the
                prefix for any given repository can be shown by using the
                ``tg --top-bases`` command and updated using the
                ``tg migrate-bases`` command).

                All of a TopGit branch's dependencies are merged into the
                corresponding TopGit base during a ``tg update`` of a branch.

        base
                See TopGit base.

        TopGit ``[PATCH]`` branch
                A TopGit branch whose subject starts with ``[PATCH]``.  By
                convention these TopGit branches contain a single patch
                (equivalent to a single patch file) and have at least one
                dependency (i.e. their ``.topdeps`` files are never empty).

        TopGit ``[BASE]`` branch
                A TopGit branch whose subject starts with ``[BASE]``.  By
                convention these TopGit branches do not actually contain
                any changes and their ``.topdeps`` files are empty.  They
                are used to control a base dependency that another set of
                branches depends on.

        TopGit ``[STAGE]`` branch
                A TopGit branch whose subject starts with ``[STAGE]``.  By
                convention these TopGit branches do not actually contain any
                changes of their own but do have one or (typically) more
                dependencies in their ``.topdeps`` file.  These branches are
                used to bring together one or (typically) more independent
                TopGit ``[PATCH]`` branches into a single branch so that
                testing and/or evaluation can be performed on the result.

        merge conflict
                When merging two (or more) heads that touch the same lines in
                the file but in different ways the result is a merge conflict
                that requires manual intervention.  If a merge conflict occurs
                with more than two heads (an octopus merge) it's generally
                replaced by multiple three-way merges so that by the time a
                user sees a merge conflict needing manual resolution, there
                will be only two heads involved.

        merge strategy
                A Git merge strategy (see the "MERGE STRATEGIES" section of
                ``git help merge``) or one of the TopGit `merge strategies`_
                used to merge two or more heads.

        TopGit merge strategy
                See the `Merge Strategies`_ section above for details but
                basically these are just in-index merges done using the
                ``git read-tree -m --aggressive`` command one or more times
                possibily assisted by the ``git merge-index`` and the
                ``git merge-file`` commands.

        octopus merge
                A merge involving more than two heads.  Note that if there are
                less than three independent heads the resulting merge that
                started out as an octopus will end up not actually being an
                octopus after all.

        out-of-date branch
                A TopGit branch is considered to be "out-of-date" when ANY of
                the following are true:

                        a) The TopGit branch does NOT contain its
                           corresponding base.

                        b) The TopGit branch does NOT contain its
                           corresponding remote branch (there may not be
                           a remote branch in which case this does not apply)

                        c) The TopGit branch's base does NOT contain its
                           corresponding remote branch's base (there may not be
                           a remote branch in which case this does not apply)

                        d) Any of the TopGit branches listed in the branch's
                           ``.topdeps`` file are NOT contained by the branch.
                           (See "branch containment" above.)

                        e) Any of the TopGit branches listed in the branch's
                           ``.topdeps`` file are out-of-date.

                Note that if a remote branch is present and is NOT out-of-date
                then it will contain its own base and (c) is mostly redundant.

        remote TopGit branch
                A Git branch with the same branch name as a TopGit branch
                but living under ``refs/remotes/<some remote>/`` instead
                of just ``refs/heads/``.

        remote TopGit base
                The TopGit base branch corresponding to a remote TopGit branch,
                which lives under ``refs/remotes/`` somewhere (depending on
                what the output of ``tg --top-bases`` is for that remote).

        three-way merge
                A three-way merge takes a common base and two heads (call them
                A and B) and creates a new file that is the common base plus
                all of the changes made between the common base and head A
                *AND* all of the changes made between the common base and
                head B.  The technique used to accomplish this is called a
                "merge strategy".


REFERENCES
----------

The following references are useful to understand the development of
topgit and its subcommands.

* tg depend:
  http://public-inbox.org/git/36ca99e90904091034m4d4d31dct78acb333612e678@mail.gmail.com/T/#u


THIRD-PARTY SOFTWARE
--------------------

The following software understands TopGit branches:

* `magit <http://magit.github.io/>`_ -- a git mode for emacs

IMPORTANT: Magit requires its topgit mode to be enabled first, as
described in its documentation, in the "Activating extensions"
subsection.  If this is not done, it will not push TopGit branches
correctly, so it's important to enable it even if you plan to mostly use
TopGit from the command line.