presubmit_support: cache the result of ChangedContents.

tests/presubmit_unittest.py

 #!/usr/bin/env python
 # 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.
 
 """Unit tests for presubmit_support.py and presubmit_canned_checks.py."""
 
 # pylint is too confused.
 # pylint: disable=E1101,E1103,R0201,W0212,W0403
 
@@ skipping 123 matching lines @@
   _INHERIT_SETTINGS = 'inherit-review-settings-ok'
 
   def testMembersChanged(self):
     self.mox.ReplayAll()
     members = [
       'AffectedFile', 'Change', 'DoGetTrySlaves', 'DoPresubmitChecks',
       'GetTrySlavesExecuter', 'GitAffectedFile',
       'GitChange', 'InputApi', 'ListRelevantPresubmitFiles', 'Main',
       'OutputApi', 'ParseFiles', 'PresubmitFailure',
       'PresubmitExecuter', 'PresubmitOutput', 'ScanSubDirs',
-      'SvnAffectedFile', 'SvnChange', 'cPickle', 'cStringIO',
+      'SvnAffectedFile', 'SvnChange', 'copy', 'cPickle', 'cStringIO',
       'fix_encoding', 'fnmatch', 'gclient_utils', 'glob', 'json',
       'load_files',
       'logging', 'marshal', 'normpath', 'optparse', 'os', 'owners', 'pickle',
       'presubmit_canned_checks', 'random', 're', 'scm', 'subprocess',
       'sys', 'tempfile', 'time', 'traceback', 'types', 'unittest', 'urllib2',
       'warn',
     ]
     # If this test fails, you should add the relevant test.
     self.compareMembers(presubmit, members)
 
@@ skipping 1964 matching lines @@
         whitelist=['^a$', '^b$'],
         blacklist=['a'])
     self.assertEqual(results, [])
     self.checkstdout(
         'Running %s\n' % presubmit.os.path.join('random_directory', 'b'))
 
 
 if __name__ == '__main__':
   import unittest
   unittest.main()