Add REST endpoint to get content of a file in a commit

It is already possible to get the content of a file from the HEAD
revision of a branch. Add the same functionality for arbitrary
commits.

Change-Id: Id8e48bb40b4f383997aa9b83440a57ab90e763ee
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-server/src/main/java/com/google/gerrit/server/project/FilesInCommitCollection.java

@@ -0,0 +1,51 @@
+// Copyright (C) 2014 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.project;
+
+import com.google.gerrit.extensions.registration.DynamicMap;
+import com.google.gerrit.extensions.restapi.ChildCollection;
+import com.google.gerrit.extensions.restapi.IdString;
+import com.google.gerrit.extensions.restapi.ResourceNotFoundException;
+import com.google.gerrit.extensions.restapi.RestView;
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+
+@Singleton
+public class FilesInCommitCollection implements
+    ChildCollection<CommitResource, FileResource> {
+  private final DynamicMap<RestView<FileResource>> views;
+
+  @Inject
+  FilesInCommitCollection(DynamicMap<RestView<FileResource>> views) {
+    this.views = views;
+  }
+
+  @Override
+  public RestView<CommitResource> list() throws ResourceNotFoundException {
+    throw new ResourceNotFoundException();
+  }
+
+  @Override
+  public FileResource parse(CommitResource parent, IdString id)
+      throws ResourceNotFoundException {
+    return new FileResource(parent.getNameKey(), parent.getCommit().getName(),
+        id.get());
+  }
+
+  @Override
+  public DynamicMap<RestView<FileResource>> views() {
+    return views;
+  }
+}