Map /diff REST API result with JSNI

Define all members of the /diff API as JSNI glue so the result can be
trivially accepted within the GWT UI.  This change includes a DiffApi
helper to build the request.

has_skip() can be used to identify files where there is missing
context and syntax highlighting might not work correctly.

text_a() and text_b() are example placeholders to recreate a file
from available lines. Performing layout with CodeMirror or another
UI system may be more complex than these functions.

Change-Id: Ie519a24725576e73b6a5ffec0326ceb30b090add

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffApi.java

@@ -0,0 +1,69 @@
+// 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.client.diff;
+
+import com.google.gerrit.client.changes.ChangeApi;
+import com.google.gerrit.client.rpc.RestApi;
+import com.google.gerrit.reviewdb.client.PatchSet;
+import com.google.gwt.user.client.rpc.AsyncCallback;
+
+public class DiffApi {
+  public enum IgnoreWhitespace {
+    NONE, TRAILING, CHANGED, ALL;
+  };
+
+  public static DiffApi diff(PatchSet.Id id, String path) {
+    return new DiffApi(ChangeApi.revision(id)
+        .view("files").id(path)
+        .view("diff"));
+  }
+
+  private final RestApi call;
+
+  private DiffApi(RestApi call) {
+    this.call = call;
+  }
+
+  public DiffApi base(PatchSet.Id id) {
+    call.addParameter("base", id.get());
+    return this;
+  }
+
+  public DiffApi ignoreWhitespace(IgnoreWhitespace w) {
+    if (w != null && w != IgnoreWhitespace.NONE) {
+      call.addParameter("ignore-whitespace", w);
+    }
+    return this;
+  }
+
+  public DiffApi intraline() {
+    call.addParameterTrue("intraline");
+    return this;
+  }
+
+  public DiffApi wholeFile() {
+    call.addParameter("context", "ALL");
+    return this;
+  }
+
+  public DiffApi context(int lines) {
+    call.addParameter("context", lines);
+    return this;
+  }
+
+  public void get(AsyncCallback<DiffInfo> cb) {
+    call.get(cb);
+  }
+}