Fix git cl format misunderstanding b/ diff prefixes

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 1946 matching lines @@
 
 def CMDformat(parser, args):
   """run clang-format on the diff"""
   CLANG_EXTS = ['.cc', '.cpp', '.h']
   parser.add_option('--full', action='store_true', default=False)
   opts, args = parser.parse_args(args)
   if args:
     parser.error('Unrecognized args: %s' % ' '.join(args))
 
   # Generate diff for the current branch's changes.
-  diff_cmd = ['diff', '--no-ext-diff']
+  diff_cmd = ['diff', '--no-ext-diff', '--no-prefix']
   if opts.full:
     # Only list the names of modified files.
     diff_cmd.append('--name-only')
   else:
     # Only generate context-less patches.
     diff_cmd.append('-U0')
 
   # Grab the merge-base commit, i.e. the upstream commit of the current
   # branch when it was created or the last time it was rebased. This is
   # to cover the case where the user may have called "git fetch origin",
@@ skipping 113 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:]))