t5517: do not assume the "matching" push is the default

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5517-push-mirror.sh

@@ -256,7 +256,7 @@ test_expect_success 'remote.foo.mirror=no has no effect' '
 		git branch keep master &&
 		git push --mirror up &&
 		git branch -D keep &&
-		git push up
+		git push up :
 	) &&
 	(
 		cd mirror &&