filter-branch: make sure orig_namespace ends with a single slash.

Later in a loop any existing ref whose path begins with it is
removed.  It would be a disaster if you allowed it to say refs/head
for example.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-filter-branch.sh

@@ -140,7 +140,7 @@ do
 		filter_subdir="$OPTARG"
 		;;
 	--original)
-		orig_namespace="$OPTARG"
+		orig_namespace=$(expr "$OPTARG/" : '\(.*[^/]\)/*$')/
 		;;
 	*)
 		usage