submodule add: fix handling of --reference=<repo> option

Doing a shift here is wrong because there is no extra
argument to consume when "--reference=<repo>" is used (note
the '=' instead of a space).

Signed-off-by: Stefan Zager <szager@google.com>
Signed-off-by: Jeff King <peff@peff.net>

git-submodule.sh

@@ -270,7 +270,6 @@ cmd_add()
 			;;
 		--reference=*)
 			reference="$1"
-			shift
 			;;
 		--)
 			shift