git recipe_module: add experimental git-cache support.

recipe_modules/git/example.py

 # Copyright 2013 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.
 
 DEPS = [
   'git',
   'recipe_engine/path',
   'recipe_engine/platform',
   'recipe_engine/properties',
   'recipe_engine/raw_io',
@@ skipping 18 matching lines @@
   retVal = api.git.checkout(
       url,
       ref=api.properties.get('revision'),
       recursive=True,
       submodule_update_force=submodule_update_force,
       set_got_revision=api.properties.get('set_got_revision'),
       curl_trace_file=curl_trace_file,
       remote_name=api.properties.get('remote_name'),
       display_fetch_size=api.properties.get('display_fetch_size'),
       file_name=api.properties.get('checkout_file_name'),
-      submodule_update_recursive=submodule_update_recursive)
+      submodule_update_recursive=submodule_update_recursive,
+      use_git_cache=api.properties.get('use_git_cache'))
 
   assert retVal == "deadbeef", (
     "expected retVal to be %r but was %r" % ("deadbeef", retVal))
 
   # count_objects shows number and size of objects in .git dir.
   api.git.count_objects(
       name='count-objects',
       can_fail_build=api.properties.get('count_objects_can_fail_build'),
       git_config_options={'foo': 'bar'})
 
@@ skipping 88 matching lines @@
       api.test('count-objects_with_bad_output_fails_build') +
       api.step_data(
           'count-objects',
           stdout=api.raw_io.output(api.git.count_objects_output('xxx'))) +
       api.properties(count_objects_can_fail_build=True))
   yield (
       api.test('cat-file_test') +
       api.step_data('git cat-file abcdef12345:TestFile',
                     stdout=api.raw_io.output('TestOutput')) +
       api.properties(revision='abcdef12345', cat_file='TestFile'))
+
+  yield (
+      api.test('git-cache-checkout') +
+      api.properties(use_git_cache=True))