GIT-VERSION-GEN: support non-standard $GIT_DIR path

make and make test both work when $GIT_DIR isn't .git, but make dist
included a bogus GIT-VERSION-FILE.

Signed-off-by: Dennis Kaarsemaker <dennis@kaarsemaker.net>
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 -o -f .git &&
+elif test -d ${GIT_DIR:-.git} -o -f .git &&
 	VN=$(git describe --match "v[0-9]*" --abbrev=7 HEAD 2>/dev/null) &&
 	case "$VN" in
 	*$LF*) (exit 1) ;;