Use three-way merge when applying Git patches.

tests/checkout_test.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.
 
 """Unit tests for checkout.py."""
 
 import logging
 import os
 import shutil
@@ skipping 439 matching lines @@
       commit_user=self.usr,
       post_processors=post_processors)
 
   def testAll(self):
     root = os.path.join(self.root_dir, self.name)
     self._check_base(self._get_co(None), root, None)
 
   def testException(self):
     self._check_exception(
         self._get_co(None),
-        'While running git apply --index -p1;\n  fatal: corrupt patch at line '
-        '12\n')
+        'While running git apply --index -3 -p1;\n  fatal: corrupt patch at '
+        'line 12\n')
 
   def testProcess(self):
     self._test_process(self._get_co)
 
   def _testPrepare(self):
     self._test_prepare(self._get_co(None))
 
   def testMove(self):
     co = self._get_co(None)
     self._check_move(co)
@@ skipping 256 matching lines @@
   if '-v' in sys.argv:
     DEBUGGING = True
     logging.basicConfig(
         level=logging.DEBUG,
         format='%(levelname)5s %(filename)15s(%(lineno)3d): %(message)s')
   else:
     logging.basicConfig(
         level=logging.ERROR,
         format='%(levelname)5s %(filename)15s(%(lineno)3d): %(message)s')
   unittest.main()