git-instaweb: allow running in a working tree subdirectory

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-instaweb.sh

@@ -20,6 +20,7 @@ start          start the web server
 restart        restart the web server
 "
 
+SUBDIRECTORY_OK=Yes
 . git-sh-setup
 
 fqgitdir="$GIT_DIR"