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>

contrib/emacs/git.el

@@ -1671,7 +1671,7 @@ Commands:
   "Entry point into git-status mode."
   (interactive "DSelect directory: ")
   (setq dir (git-get-top-dir dir))
-  (if (file-directory-p (concat (file-name-as-directory dir) ".git"))
+  (if (file-exists-p (concat (file-name-as-directory dir) ".git"))
       (let ((buffer (or (and git-reuse-status-buffer (git-find-status-buffer dir))
                         (create-file-buffer (expand-file-name "*git-status*" dir)))))
         (switch-to-buffer buffer)