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

patch.py

 # coding=utf8
 # 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.
 """Utility functions to handle patches."""
 
 import posixpath
 import os
 import re
 
@@ skipping 300 matching lines @@
 
   def mangle(self, string):
     """Mangle a file path."""
     return '/'.join(string.replace('\\', '/').split('/')[self.patchlevel:])
 
   def _verify_git_header(self):
     """Sanity checks the header.
 
     Expects the following format:
 
-    <garbagge>
+    <garbage>
     diff --git (|a/)<filename> (|b/)<filename>
     <similarity>
     <filemode changes>
     <index>
     <copy|rename from>
     <copy|rename to>
     --- <filename>
     +++ <filename>
 
     Everything is optional except the diff --git line.
@@ skipping 202 matching lines @@
   def __iter__(self):
     for patch in self.patches:
       yield patch
 
   def __getitem__(self, key):
     return self.patches[key]
 
   @property
   def filenames(self):
     return [p.filename for p in self.patches]