worktree: improve message when creating a new worktree

worktree: improve message when creating a new worktree

Currently 'git worktree add' produces output like the following:

    Preparing ../foo (identifier foo)
    HEAD is now at 26da330922 <title>

The '../foo' is the path where the worktree is created, which the user
has just given on the command line.  The identifier is an internal
implementation detail, which is not particularly relevant for the user
and indeed isn't mentioned explicitly anywhere in the man page.

Instead of this message, print a message that gives the user a bit more
detail of what exactly 'git worktree' is doing.  There are various dwim
modes which perform some magic under the hood, which should be
helpful to users.  Just from the output of the command it is not always
visible to users what exactly has happened.

Help the users a bit more by modifying the "Preparing ..." message and
adding some additional information of what 'git worktree add' did under
the hood, while not displaying the identifier anymore.

Currently there are several different cases:

  - 'git worktree add -b ...' or 'git worktree add <path>', both of
    which create a new branch, either through the user explicitly
    requesting it, or through 'git worktree add' implicitly creating
    it.  This will end up with the following output:

      Preparing worktree (new branch '<branch>')
      HEAD is now at 26da330922 <title>

  - 'git worktree add -B ...', which may either create a new branch if
    the branch with the given name does not exist yet, or resets an
    existing branch to the current HEAD, or the commit-ish given.
    Depending on which action is taken, we'll end up with the following
    output:

      Preparing worktree (resetting branch '<branch>'; was at caa68db14)
      HEAD is now at 26da330922 <title>

    or:

      Preparing worktree (new branch '<branch>')
      HEAD is now at 26da330922 <title>

  - 'git worktree add --detach' or 'git worktree add <path>
    <commit-ish>', both of which create a new worktree with a detached
    HEAD, for which we will print the following output:

      Preparing worktree (detached HEAD 26da330922)
      HEAD is now at 26da330922 <title>

  - 'git worktree add <path> <local-branch>', which checks out the
    branch and prints the following output:

      Preparing worktree (checking out '<local-branch>')
      HEAD is now at 47007d5 <title>

Additionally currently the "Preparing ..." line is printed to stderr,
while the "HEAD is now at ..." line is printed to stdout by 'git reset
--hard', which is used internally by 'git worktree add'.  Fix this
inconsistency by printing the "Preparing ..." message to stdout as
well.  As "Preparing ..." is not an error, stdout also seems like the
more appropriate output stream.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>