Added support for relative paths for the .gclient cache_dir field.

gclient.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.
 
 """Meta checkout manager supporting both Subversion and GIT."""
 # Files
 #   .gclient      : Current client configuration, written by 'config' command.
 #                   Format is a Python script defining 'solutions', a list whose
 #                   entries each are maps binding the strings "name" and "url"
@@ skipping 1144 matching lines @@
     except SyntaxError, e:
       gclient_utils.SyntaxErrorToError('.gclient', e)
 
     # Append any target OS that is not already being enforced to the tuple.
     target_os = config_dict.get('target_os', [])
     if config_dict.get('target_os_only', False):
       self._enforced_os = tuple(set(target_os))
     else:
       self._enforced_os = tuple(set(self._enforced_os).union(target_os))
 
-    gclient_scm.GitWrapper.cache_dir = config_dict.get('cache_dir')
-    git_cache.Mirror.SetCachePath(config_dict.get('cache_dir'))
+    cache_dir = config_dict.get('cache_dir')
+    if cache_dir:
+      cache_dir = os.path.join(self.root_dir, cache_dir)
+      cache_dir = os.path.abspath(cache_dir)
+    gclient_scm.GitWrapper.cache_dir = cache_dir
+    git_cache.Mirror.SetCachePath(cache_dir)
 
     if not target_os and config_dict.get('target_os_only', False):
       raise gclient_utils.Error('Can\'t use target_os_only if target_os is '
                                 'not specified')
 
     deps_to_add = []
     for s in config_dict.get('solutions', []):
       try:
         deps_to_add.append(Dependency(
             self, s['name'], s['url'],
@@ skipping 880 matching lines @@
     print >> sys.stderr, 'Error: %s' % str(e)
     return 1
   finally:
     gclient_utils.PrintWarnings()
 
 
 if '__main__' == __name__:
   sys.exit(Main(sys.argv[1:]))
 
 # vim: ts=2:sw=2:tw=80:et: