Add a git cache for gclient sync operations.

gclient.py

Index: gclient.py
diff --git a/gclient.py b/gclient.py
index 1f4801b9464af43a0bf9e252cde59053de0e23bf..762e3142058915c4ebb3ae1087c550f28ac2175f 100755
--- a/gclient.py
+++ b/gclient.py
@@ -66,6 +66,7 @@ Specifying a target OS
 
 __version__ = "0.6.4"
 
+import collections
 import copy
 import logging
 import optparse
@@ -75,6 +76,7 @@ import posixpath
 import pprint
 import re
 import sys
+import threading
 import urllib
 import urlparse
 
@@ -1543,9 +1545,18 @@ def CMDsync(parser, args):
                          'actual HEAD revision from the repository')
   parser.add_option('--upstream', action='store_true',
                     help='Make repo state match upstream branch.')
+  parser.add_option('--cache-dir',
+                    help='For git, cache all repos into this dir and do shared '
+                         'clones from the cache, instead of cloning directly '
+                         'from the remote. (experimental)')
   (options, args) = parser.parse_args(args)
   client = GClient.LoadCurrentConfig(options)
 
+  if options.cache_dir:
+    # If a given cache is used in a solution more than once, prevent multiple
+    # threads from updating it simultaneously.
+    options.cache_locks = collections.defaultdict(threading.Lock)
+
   if not client:
     raise gclient_utils.Error('client not configured; see \'gclient config\'')