Add an event for the update of a project's HEAD

When the HEAD of a project is updated Gerrit now informs about this by
sending an event. Plugins can implement a listener to be notified
about this event. Having this new event enables the replication plugin
to replicate HEAD updates.

Change-Id: I4dd7969020880190668190e8345197a23dbc92f1
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/events/HeadUpdatedListener.java

@@ -0,0 +1,29 @@
+// Copyright (C) 2013 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.extensions.events;
+
+import com.google.gerrit.extensions.annotations.ExtensionPoint;
+
+/** Notified whenever the HEAD of a project is updated. */
+@ExtensionPoint
+public interface HeadUpdatedListener {
+  public interface Event {
+    String getProjectName();
+    String getOldHeadName();
+    String getNewHeadName();
+  }
+
+  void onHeadUpdated(Event event);
+}