completion: avoid user confusion in non-cone mode

completion: avoid user confusion in non-cone mode

It is tempting to think of "files and directories" of the current
directory as valid inputs to the add and set subcommands of git
sparse-checkout.  However, in non-cone mode, they often aren't and using
them as potential completions leads to *many* forms of confusion:

Issue #1. It provides the *wrong* files and directories.

For
    git sparse-checkout add
we always want to add files and directories not currently in our sparse
checkout, which means we want file and directories not currently present
in the current working tree.  Providing the files and directories
currently present is thus always wrong.

For
    git sparse-checkout set
we have a similar problem except in the subset of cases where we are
trying to narrow our checkout to a strict subset of what we already
have.  That is not a very common scenario, especially since it often
does not even happen to be true for the first use of the command; for
years we required users to create a sparse-checkout via
    git sparse-checkout init
    git sparse-checkout set <args...>
(or use a clone option that did the init step for you at clone time).
The init command creates a minimal sparse-checkout with just the
top-level directory present, meaning the set command has to be used to
expand the checkout.  Thus, only in a special and perhaps unusual cases
would any of the suggestions from normal file and directory completion
be appropriate.

Issue #2: Suggesting patterns that lead to warnings is unfriendly.

If the user specifies any regular file and omits the leading '/', then
the sparse-checkout command will warn the user that their command is
problematic and suggest they use a leading slash instead.

Issue #3: Completion gets confused by leading '/', and provides wrong paths.

Users often want to anchor their patterns to the toplevel of the
repository, especially when listing individual files.  There are a
number of reasons for this, but notably even sparse-checkout encourages
them to do so (as noted above).  However, if users do so (via adding a
leading '/' to their pattern), then bash completion will interpret the
leading slash not as a request for a path at the toplevel of the
repository, but as a request for a path at the root of the filesytem.
That means at best that completion cannot help with such paths, and if
it does find any completions, they are almost guaranteed to be wrong.

Issue #4: Suggesting invalid patterns from subdirectories is unfriendly.

There is no per-directory equivalent to .gitignore with
sparse-checkouts.  There is only a single worktree-global
$GIT_DIR/info/sparse-checkout file.  As such, paths to files must be
specified relative to the toplevel of a repository.  Providing
suggestions of paths that are relative to the current working directory,
as bash completion defaults to, is wrong when the current working
directory is not the worktree toplevel directory.

Issue #5: Paths with special characters will be interpreted incorrectly

The entries in the sparse-checkout file are patterns, not paths.  While
most paths also qualify as patterns (though even in such cases it would
be better for users to not use them directly but prefix them with a
leading '/'), there are a variety of special characters that would need
special escaping beyond the normal shell escaping: '*', '?', '\', '[',
']', and any leading '#' or '!'.  If completion suggests any such paths,
users will likely expect them to be treated as an exact path rather than
as a pattern that might match some number of files other than 1.

However, despite the first four issues, we can note that _if_ users are
using tab completion, then they are probably trying to specify a path in
the index.  As such, we transform their argument into a top-level-rooted
pattern that matches such a file.  For example, if they type:
   git sparse-checkout add Make<TAB>
we could "complete" to
   git sparse-checkout add /Makefile
or, if they ran from the Documentation/technical/ subdirectory:
   git sparse-checkout add m<TAB>
we could "complete" it to:
   git sparse-checkout add /Documentation/technical/multi-pack-index.txt
Note in both cases I use "complete" in quotes, because we actually add
characters both before and after the argument in question, so we are
kind of abusing "bash completions" to be "bash completions AND
beginnings".

The fifth issue is a bit stickier, especially when you consider that we
not only need to deal with escaping issues because of special meanings
of patterns in sparse-checkout & gitignore files, but also that we need
to consider escaping issues due to ls-files needing to sometimes quote
or escape characters, and because the shell needs to escape some
characters.  The multiple interacting forms of escaping could get ugly;
this patch makes no attempt to do so and simply documents that we
decided to not deal with those corner cases for now but at least get the
common cases right.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>