Add UploadPackInitializer extension point

I suggested to Luca at the hackathon that he could use this mechanism to
limit refs advertised to Jenkins, instead of writing a custom
PermissionBackend. Getting this in 2.15 means people using his plugin
won't have to wait for the next release to stop using the
PermissionBackend hack.

Change-Id: I8c38eef94d6e505b926b3da6c470e34f6613ca2c

gerrit-server/src/main/java/com/google/gerrit/server/git/UploadPackInitializer.java

@@ -0,0 +1,35 @@
+// Copyright (C) 2017 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.git;
+
+import com.google.gerrit.extensions.annotations.ExtensionPoint;
+import com.google.gerrit.reviewdb.client.Project;
+import org.eclipse.jgit.transport.UploadPack;
+
+@ExtensionPoint
+public interface UploadPackInitializer {
+
+  /**
+   * UploadPack initialization.
+   *
+   * <p>Invoked by Gerrit when a new UploadPack instance is created and just before it is used.
+   * Implementors will usually call setXXX methods on the uploadPack parameter in order to set
+   * additional properties on it.
+   *
+   * @param project project for which the UploadPack is created
+   * @param uploadPack the UploadPack instance which is being initialized
+   */
+  void init(Project.NameKey project, UploadPack uploadPack);
+}