git-gui: use --absolute-git-dir

git-gui: use --absolute-git-dir

git-gui uses git rev-parse --git-dir to get the pathname of the
discovered git repository. The returned value can be relative, and is
'.' if the current directory is the top of the repository directory
itself.  git-gui has code to change '.' to [pwd] in this case so that
subsequent logic runs.

But, git rev-parse supports --absolute-git-dir from fac60b8925
("rev-parse: add option for absolute or relative path formatting",
2020-12-13), and included in git 2.31. git-gui requires git >= 2.36, so
this more useful form is always available. Use --absolute-git-dir to
always get an absolute path, avoiding the need for other checks, and
delete the now unneeded code to fix a relative _gitdir.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>