Add script to apply a patch from rietveld.

patch.py

 # coding=utf8
 # Copyright (c) 2011 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.
 """Utility functions to handle patches."""
 
 import posixpath
 import os
 import re
 
@@ skipping 71 matching lines @@
 
   def get(self):
     return self.data
 
 
 class FilePatchDiff(FilePatchBase):
   """Patch for a single file."""
 
   def __init__(self, filename, diff, svn_properties):
     super(FilePatchDiff, self).__init__(filename)
+    if not diff:
+      self._fail('File doesn\'t have a diff.')
     self.diff_header, self.diff_hunks = self._split_header(diff)
     self.svn_properties = svn_properties or []
     self.is_git_diff = self._is_git_diff_header(self.diff_header)
     self.patchlevel = 0
     if self.is_git_diff:
       self._verify_git_header()
       assert not svn_properties
     else:
       self._verify_svn_header()
 
@@ skipping 169 matching lines @@
     for patch in self.patches:
       patch.set_relpath(relpath)
 
   def __iter__(self):
     for patch in self.patches:
       yield patch
 
   @property
   def filenames(self):
     return [p.filename for p in self.patches]