Support GET, PUT, DELETE on /changes/{id}/topic

The topic is now modified by REST API calls against /topic. The web UI
uses PUT to alter the topic with a JSON payload, but the server
accepts quite a few different REST forms that clients can easily send.

Change-Id: Ia5edbb232bc288acdad8b145956ad275d170629a

gerrit-server/src/main/java/com/google/gerrit/server/change/GetTopic.java

@@ -0,0 +1,26 @@
+// Copyright (C) 2012 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.change;
+
+import com.google.common.base.Strings;
+import com.google.gerrit.extensions.restapi.RestReadView;
+import com.google.gwtorm.server.OrmException;
+
+class GetTopic implements RestReadView<ChangeResource> {
+  @Override
+  public Object apply(ChangeResource rsrc) throws OrmException {
+    return Strings.nullToEmpty(rsrc.getChange().getTopic());
+  }
+}