emacs: make 'git-status' work with separate git dirs

emacs: make 'git-status' work with separate git dirs

when trying 'M-x git-status' in a submodule created with recent (1.7.5+)
git, the command fails with

| ... is not a git working tree

This is caused by creating submodules with '--separate-git-dir' but
still checking for a working tree by testing for a '.git' directory.

The patch fixes this by relaxing the existing detection a little bit.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Acked-by: Alexandre Julliard <julliard@winehq.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>