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>