[Predator] Add local cache for get command output.

appengine/findit/util_scripts/test/script_util_test.py

 # Copyright 2016 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.
 
 import subprocess
 
 import script_util
 from testing_utils import testing
 
 
 class ScriptUtilTest(testing.AppengineTestCase):
 
+  # TODO(katesonia): Figure out a good way to work around cache.
   def testGetLocalGitCommandOutput(self):
     class _MockProcess(object):
       def __init__(self, command, *_):
         self.command = command
 
       def communicate(self, *_):
-        return self.command, 'dummy'
+        return self.command, 'error'
 
       @property
       def returncode(self):
         return 1 if self.command == 'dummy' else 0
 
     self.mock(subprocess, 'Popen', lambda command, **_: _MockProcess(command))
     output = script_util.GetCommandOutput('command')
     self.assertEqual(output, 'command')
 
     output = script_util.GetCommandOutput('dummy')
     self.assertEqual(output, None)