Add a warning when git-cl-upload-hook is modified to update git-cl accordingly.

PRESUBMIT.py

 # 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.
 
 """Top-level presubmit script for depot tools.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts for
 details on the presubmit API built into gcl.
 """
 
 UNIT_TESTS = [
   'tests.gcl_unittest',
   'tests.gclient_scm_test',
   'tests.gclient_utils_test',
   'tests.presubmit_unittest',
   'tests.scm_unittest',
   'tests.trychange_unittest',
   'tests.watchlists_unittest',
 ]
 
 def CheckChangeOnUpload(input_api, output_api):
   output = []
   output.extend(input_api.canned_checks.RunPythonUnitTests(input_api,
                                                            output_api,
                                                            UNIT_TESTS))
+  output.extend(WasGitClUploadHookModified(input_api, output_api))
   return output
 
 
 def CheckChangeOnCommit(input_api, output_api):
   output = []
   output.extend(input_api.canned_checks.RunPythonUnitTests(input_api,
                                                            output_api,
                                                            UNIT_TESTS))
   output.extend(input_api.canned_checks.CheckDoNotSubmit(input_api,
                                                          output_api))
+  output.extend(WasGitClUploadHookModified(input_api, output_api))
   return output
+
+def WasGitClUploadHookModified(input_api, output_api):
+  for affected_file in input_api.AffectedSourceFiles(None):
+    if (input_api.os_path.basename(affected_file.LocalPath()) ==
+        'git-cl-upload-hook'):
+      return [output_api.PresubmitPromptWarning(
+          'Don\'t forget to fix git-cl to download the newest version of '
+          'git-cl-upload-hook')]
+  return []