Git.pm: use "rev-parse --absolute-git-dir" rather than perl code

Git.pm: use "rev-parse --absolute-git-dir" rather than perl code

When we open a repository with the "Directory" option, we use "rev-parse
--git-dir" to get the path relative to that directory, and then use
Cwd::abs_path() to make it absolute (since our process working directory
may not be the same).

These days we can just ask for "--absolute-git-dir" instead, which saves
us a little code. That option was added in Git v2.13.0 via a2f5a87626
(rev-parse: add '--absolute-git-dir' option, 2017-02-03). I don't think
we make any promises about running mismatched versions of git and
Git.pm, but even if somebody tries it, that's sufficiently old that it
should be OK.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>