Handle the case of git cl lint when the CL is empty.

git_cl.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 # Copyright (C) 2008 Evan Martin <martine@danga.com>
 
 """A git-command for integrating reviews on Rietveld."""
 
 import datetime
@@ skipping 1395 matching lines @@
     return 1
 
   # Change the current working directory before calling lint so that it
   # shows the correct base.
   previous_cwd = os.getcwd()
   os.chdir(settings.GetRoot())
   try:
     cl = Changelist()
     change = cl.GetChange(cl.GetCommonAncestorWithUpstream(), None)
     files = [f.LocalPath() for f in change.AffectedFiles()]
+    if not files:
+      print "Cannot lint an empty CL"
+      return 1
 
     # Process cpplints arguments if any.
     command = args + files
     if options.filter:
       command = ['--filter=' + ','.join(options.filter)] + command
     filenames = cpplint.ParseArguments(command)
 
     white_regex = re.compile(settings.GetLintRegex())
     black_regex = re.compile(settings.GetLintIgnoreRegex())
     extra_check_functions = [cpplint_chromium.CheckPointerDeclarationWhitespace]
@@ skipping 1174 matching lines @@
         ('AppEngine is misbehaving and returned HTTP %d, again. Keep faith '
           'and retry or visit go/isgaeup.\n%s') % (e.code, str(e)))
 
 
 if __name__ == '__main__':
   # These affect sys.stdout so do it outside of main() to simplify mocks in
   # unit testing.
   fix_encoding.fix_encoding()
   colorama.init()
   sys.exit(main(sys.argv[1:]))