Add pylint to presubmit checks

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_smoketest',
   '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))
+  output.extend(RunPylint(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))
+  output.extend(RunPylint(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 []
+
+def RunPylint(input_api, output_api):
+  import glob
+  files = glob.glob('*.py')
+  # It's a python script
+  files.append('git-try')
+  # It uses non-standard pylint exceptions that makes pylint always fail.
+  files.remove('cpplint.py')
+  proc = input_api.subprocess.Popen(['pylint', '-E'] + files)
+  proc.communicate()
+  if proc.returncode:
+    return [output_api.PresubmitError('Fix pylint errors first.')]
+  return []