Get presubmit_support to compute diffs against a given upstream.

git_cl.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.
 
 # Copyright (C) 2008 Evan Martin <martine@danga.com>
 
 """A git-command for integrating reviews on Rietveld."""
 
 from distutils.version import LooseVersion
@@ skipping 777 matching lines @@
 
     if not author:
       author = RunGit(['config', 'user.email']).strip() or None
     return presubmit_support.GitChange(
         name,
         description,
         absroot,
         files,
         issue,
         patchset,
-        author)
+        author,
+        upstream=upstream_branch)
 
   def RunHook(self, committing, may_prompt, verbose, change):
     """Calls sys.exit() if the hook fails; returns a HookResults otherwise."""
 
     try:
       return presubmit_support.DoPresubmitChecks(change, committing,
           verbose=verbose, output_stream=sys.stdout, input_stream=sys.stdin,
           default_presubmit=None, may_prompt=may_prompt,
           rietveld_obj=self.RpcServer())
     except presubmit_support.PresubmitFailure, e:
@@ skipping 1757 matching lines @@
         ('AppEngine is misbehaving and returned HTTP %d, again. Keep faith '
           'and retry or visit go/isgaeup.\n%s') % (e.code, str(e)))
 
 
 if __name__ == '__main__':
   # These affect sys.stdout so do it outside of main() to simplify mocks in
   # unit testing.
   fix_encoding.fix_encoding()
   colorama.init()
   sys.exit(main(sys.argv[1:]))