transport-helper: barf when user tries old:new

Otherwise with certain remote helpers (the ones that support 'export'),
the users will be pushing to the wrong branch:

  git push topic:master

Will push the topic branch, as if the user typed:

  git push topic

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

transport-helper.c

@@ -813,9 +813,11 @@ static int push_refs_with_export(struct transport *transport,
 			die("remote-helpers do not support ref deletion");
 		}
 
-		if (ref->peer_ref)
+		if (ref->peer_ref) {
+			if (strcmp(ref->peer_ref->name, ref->name))
+				die("remote-helpers do not support old:new syntax");
 			string_list_append(&revlist_args, ref->peer_ref->name);
-
+		}
 	}
 
 	if (get_exporter(transport, &exporter, &revlist_args))