git-gui: work from the .git dir

When git-gui is run from a .git dir, _gitdir would be set to "." by
rev-parse, something that confuses the worktree detection.

Fix by expanding the value of _gitdir to pwd in this special case.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>