Teach GIT-VERSION-GEN about the .git file

The presence of a .git directory used to be good enough evidence that
GIT-VERSION-GEN could use 'git describe' to get a version number. But
now .git might as well be a file so the test must be extended to cater for
such setups.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

GIT-VERSION-GEN

@@ -11,7 +11,7 @@ LF='
 if test -f version
 then
 	VN=$(cat version) || VN="$DEF_VER"
-elif test -d .git &&
+elif test -d .git -o -f .git &&
 	VN=$(git describe --abbrev=4 HEAD 2>/dev/null) &&
 	case "$VN" in
 	*$LF*) (exit 1) ;;