presubmit: Call 'git diff' once per change instead of once per file

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."""
 
 import json
@@ skipping 678 matching lines @@
     # We use the sha1 of HEAD as a name of this change.
     name = RunCommand(['git', '--no-pager', 'rev-parse', 'HEAD']).strip()
     # Need to pass a relative path for msysgit.
     try:
       files = scm.GIT.CaptureStatus([root], '.', upstream_branch)
     except subprocess2.CalledProcessError:
       DieWithError(
           ('\nFailed to diff against upstream branch %s!\n\n'
            'This branch probably doesn\'t exist anymore. To reset the\n'
            'tracking branch, please run\n'
-           '    git branch --set-upstream %s trunk\n'
+           '    git branch --set-upstream-to=trunk\n'
            'replacing trunk with origin/master or the relevant branch') %
-          (upstream_branch, self.GetBranch()))
+          (upstream_branch))
 
     issue = self.GetIssue()
     patchset = self.GetPatchset()
     if issue:
       description = self.GetDescription()
     else:
       # If the change was never uploaded, use the log messages of all commits
       # up to the branch point, as git cl upload will prefill the description
       # with these log messages.
       description = RunCommand(['git', '--no-pager',
@@ skipping 1384 matching lines @@
   GenUsage(parser, 'help')
   return CMDhelp(parser, argv)
 
 
 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:]))