transport-helper: add support for old:new refspec

By using fast-export's new --refspec option.

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

t/t5801-remote-helpers.sh

@@ -87,7 +87,7 @@ test_expect_success 'push new branch by name' '
 	compare_refs local HEAD server refs/heads/new-name
 '
 
-test_expect_failure 'push new branch with old:new refspec' '
+test_expect_success 'push new branch with old:new refspec' '
 	(cd local &&
 	 git push origin new-name:new-refspec
 	) &&

transport-helper.c

@@ -856,7 +856,7 @@ static int push_refs_with_export(struct transport *transport,
 	struct ref *ref;
 	struct child_process *helper, exporter;
 	struct helper_data *data = transport->data;
-	struct string_list revlist_args = STRING_LIST_INIT_NODUP;
+	struct string_list revlist_args = STRING_LIST_INIT_DUP;
 	struct strbuf buf = STRBUF_INIT;
 
 	if (!data->refspecs)
@@ -894,15 +894,22 @@ static int push_refs_with_export(struct transport *transport,
 		free(private);
 
 		if (ref->peer_ref) {
-			if (strcmp(ref->peer_ref->name, ref->name))
-				die("remote-helpers do not support old:new syntax");
+			if (strcmp(ref->name, ref->peer_ref->name)) {
+				struct strbuf buf = STRBUF_INIT;
+				strbuf_addf(&buf, "%s:%s", ref->peer_ref->name, ref->name);
+				string_list_append(&revlist_args, "--refspec");
+				string_list_append(&revlist_args, buf.buf);
+				strbuf_release(&buf);
+			}
 			string_list_append(&revlist_args, ref->peer_ref->name);
 		}
 	}
 
 	if (get_exporter(transport, &exporter, &revlist_args))
 		die("Couldn't run fast-export");
 
+	string_list_clear(&revlist_args, 1);
+
 	if (finish_command(&exporter))
 		die("Error while running fast-export");
 	push_update_refs_status(data, remote_refs, flags);