GIT-VERSION-GEN: do not fail if a 'HEAD' file exists in the working copy

Signed-off-by: Lea Wiemann <LeWiemann@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/GIT-VERSION-GEN b/GIT-VERSION-GEN
index f221447..cb7cd4b 100755
--- a/GIT-VERSION-GEN
+++ b/GIT-VERSION-GEN
@@ -16,7 +16,7 @@
 	case "$VN" in
 	*$LF*) (exit 1) ;;
 	v[0-9]*)
-		test -z "$(git diff-index --name-only HEAD)" ||
+		test -z "$(git diff-index --name-only HEAD --)" ||
 		VN="$VN-dirty" ;;
 	esac
 then