remote-helpers: allow requesing the path to the .git directory

remote-helpers: allow requesing the path to the .git directory

The 'gitdir' capability is reported by the remote helper if it
requires the location of the .git directory. The location of the .git
directory can then be used by the helper to store status files even
when the current directory is not a git repository (such as is the
case when cloning).

The location of the .git dir is specified as an absolute path.

Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>