Support getting changes of non-HEAD branches

tests/presubmit_unittest.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 presubmit_support.py and presubmit_canned_checks.py."""
 
 # pylint: disable=E1101,E1103
 
 import StringIO
@@ skipping 495 matching lines @@
 
     for op, path in files:
       full_path = presubmit.os.path.join(self.fake_root_dir, *path.split('/'))
       if op.startswith('D'):
         os.path.exists(full_path).AndReturn(False)
       else:
         os.path.exists(full_path).AndReturn(False)
         os.path.isfile(full_path).AndReturn(True)
 
     presubmit.scm.GIT.GenerateDiff(
-        self.fake_root_dir, files=[], full_move=True, branch=None
-        ).AndReturn('\n'.join(unified_diff))
+        self.fake_root_dir, files=[], full_move=True, branch=None,
+        branch_head='HEAD',).AndReturn('\n'.join(unified_diff))
 
     self.mox.ReplayAll()
 
     change = presubmit.GitChange(
         'mychange',
         '\n'.join(description_lines),
         self.fake_root_dir,
         files,
         0,
         0,
@@ skipping 2360 matching lines @@
         owners_check=False)
     self.assertEqual(2, len(results))
     self.assertEqual(
         'Found line ending with white spaces in:', results[0]._message)
     self.checkstdout('')
 
 
 if __name__ == '__main__':
   import unittest
   unittest.main()