rev-parse: add option --resolve-git-dir <path>

rev-parse: add option --resolve-git-dir <path>

Check if <path> is a valid git-dir or a valid git-file that points
to a valid git-dir.

We want tests to be independent from the fact that a git-dir may
be a git-file. Thus we changed tests to use this feature.

Signed-off-by: Fredrik Gustafsson <iveqy@iveqy.com>
Mentored-by: Jens Lehmann <Jens.Lehmann@web.de>
Mentored-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>