Add the tree check there instead of implementing it in the root PRESUBMIT.py.

PRESUBMIT_unittest.py

 #!/usr/bin/python
 # Copyright (c) 2009 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 top-level Chromium presubmit script.
 """
 
 
 import os
@@ skipping 22 matching lines @@
   def __init__(self, path, action='A'):
     self.path = path
     self.action = action
 
   def Action(self):
     return self.action
 
   def LocalPath(self):
     return self.path
 
+  def AbsoluteLocalPath(self):
+    return self.path
+
 
 class MockOutputApi(object):
   class PresubmitError(object):
     def __init__(self, msg, items=[], long_text=''):
       self.msg = msg
       self.items = items
 
 
 class PresubmitUnittest(unittest.TestCase):
   def setUp(self):
@@ skipping 41 matching lines @@
     api = MockInputApi()
     api.affected_files = [
       MockAffectedFile('foo/blat/source.py', 'D'),
     ]
     self.file_contents = 'file with \n\terror\nhere\r\nyes there'
     self.failUnless(len(PRESUBMIT.LocalChecks(api, MockOutputApi)) == 0)
 
 
 if __name__ == '__main__':
   unittest.main()