Fix "git cl format" when newer clang-format version is installed.

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 2249 matching lines @@
       return 0
     RunCommand(['clang-format', '-i', '-style', 'Chromium'] + files,
                cwd=top_dir)
   else:
     # diff_output is a patch to send to clang-format-diff.py
     cfd_path = os.path.join('/usr', 'lib', 'clang-format',
                             'clang-format-diff.py')
     if not os.path.exists(cfd_path):
       DieWithError('Could not find clang-format-diff at %s.' % cfd_path)
     cmd = [sys.executable, cfd_path, '-p0', '-style', 'Chromium']
+
+    # Newer versions of clang-format-diff.py require an explicit -i flag
+    # to apply the edits to files, otherwise it just displays a diff.
+    # Probe the usage string to verify if this is needed.
+    help_text = RunCommand([sys.executable, cfd_path, '-h'])
+    if '[-i]' in help_text:
+      cmd.append('-i')
+
     RunCommand(cmd, stdin=diff_output, cwd=top_dir)
 
   return 0
 
 
 class OptionParser(optparse.OptionParser):
   """Creates the option parse and add --verbose support."""
   def __init__(self, *args, **kwargs):
     optparse.OptionParser.__init__(
         self, *args, prog='git cl', version=__version__, **kwargs)
@@ skipping 30 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:]))