Add a git cache for gclient sync operations.

tests/gclient_scm_test.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 """Unit tests for gclient_scm.py."""
 
 # pylint: disable=E1103
 
 # Import before super_mox to keep valid references.
@@ skipping 667 matching lines @@
   class OptionsObject(object):
     def __init__(self, verbose=False, revision=None):
       self.verbose = verbose
       self.revision = revision
       self.manually_grab_svn_rev = True
       self.deps_os = None
       self.force = False
       self.reset = False
       self.nohooks = False
       self.upstream = False
+      self.cache_dir = None
       self.merge = False
       self.jobs = 1
       self.delete_unversioned_trees = False
 
   sample_git_import = """blob
 mark :1
 data 6
 Hello
 
 blob
@@ skipping 495 matching lines @@
 
 if __name__ == '__main__':
   if '-v' in sys.argv:
     logging.basicConfig(
         level=logging.DEBUG,
         format='%(asctime).19s %(levelname)s %(filename)s:'
                '%(lineno)s %(message)s')
   unittest.main()
 
 # vim: ts=2:sw=2:tw=80:et: