Be nicer if git executable is not installed

This patch avoids problems if vc-git.el is installed and activated, but
the git executable is not available, for example
http://list-archive.xemacs.org/xemacs-beta/200608/msg00062.html

Signed-off-by: Ville Skyttä <scop@xemacs.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

contrib/emacs/vc-git.el

@@ -54,7 +54,7 @@
     (let* ((dir (file-name-directory file))
            (name (file-relative-name file dir)))
       (when dir (cd dir))
-      (and (eq 0 (call-process "git" nil '(t nil) nil "ls-files" "-c" "-z" "--" name))
+      (and (ignore-errors (eq 0 (call-process "git" nil '(t nil) nil "ls-files" "-c" "-z" "--" name)))
            (let ((str (buffer-string)))
              (and (> (length str) (length name))
                   (string= (substring str 0 (1+ (length name))) (concat name "\0"))))))))