rev-parse: add '--absolute-git-dir' option

rev-parse: add '--absolute-git-dir' option

The output of 'git rev-parse --git-dir' can be either a relative or an
absolute path, depending on whether the current working directory is
at the top of the worktree or the .git directory or not, or how the
path to the repository is specified via the '--git-dir=<path>' option
or the $GIT_DIR environment variable.  And if that output is a
relative path, then it is relative to the directory where any 'git
-C <path>' options might have led us.

This doesn't matter at all for regular scripts, because the git
wrapper automatically takes care of changing directories according to
the '-C <path>' options, and the scripts can then simply follow any
path returned by 'git rev-parse --git-dir', even if it's a relative
path.

Our Bash completion script, however, is unique in that it must run
directly in the user's interactive shell environment.  This means that
it's not executed through the git wrapper and would have to take care
of any '-C <path> options on its own, and it can't just change
directories as it pleases.  Consequently, adding support for taking
any '-C <path>' options on the command line into account during
completion turned out to be considerably more difficult, error prone
and required more subshells and git processes when it had to cope with
a relative path to the .git directory.

Help this rather special use case and teach 'git rev-parse' a new
'--absolute-git-dir' option which always outputs a canonicalized
absolute path to the .git directory, regardless of whether the path is
discovered automatically or is specified via $GIT_DIR or 'git
--git-dir=<path>'.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>