Add apply_gerrit_ref to bot_update api

recipe_modules/bot_update/resources/apply_gerrit.py

+#!/usr/bin/env python
+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import optparse
+import sys
+
+import bot_update  # pylint: disable=relative-import
+
+
+if __name__ == '__main__':
+  parse = optparse.OptionParser()
+
+  parse.add_option('--gerrit_repo',
+                   help='Gerrit repository to pull the ref from.')
+  parse.add_option('--gerrit_ref', help='Gerrit ref to apply.')
+  parse.add_option('--root', help='The location of the checkout.')
+  parse.add_option('--gerrit_no_reset', action='store_true',
+                   help='Bypass calling reset after applying a gerrit ref.')
+  parse.add_option('--gerrit_rebase_patch_ref', action='store_true',
+                   help='Rebase Gerrit patch ref after of checking it out.')
+
+  options, _ = parse.parse_args()
+
+  sys.exit(
+      bot_update.apply_gerrit_ref(
+          options.gerrit_repo,
+          options.gerrit_ref,
+          options.root,
+          not options.gerrit_no_reset,
+          options.gerrit_rebase_patch_ref)
+  )