Fix pylint presubmit failure

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 difflib
@@ skipping 72 matching lines @@
                                         env=env,
                                         stdout=subprocess2.PIPE)
     return code, out[0]
   except ValueError:
     # When the subprocess fails, it returns None.  That triggers a ValueError
     # when trying to unpack the return value into (out, code).
     return 1, ''
 
 
 def IsGitVersionAtLeast(min_version):
-  PREFIX='git version '
+  prefix = 'git version '
   version = RunGit(['--version']).strip()
-  return (version.startswith(PREFIX) and
-      LooseVersion(version[len(PREFIX):]) >= LooseVersion(min_version))
+  return (version.startswith(prefix) and
+      LooseVersion(version[len(prefix):]) >= LooseVersion(min_version))
 
 
 def usage(more):
   def hook(fn):
     fn.usage_more = more
     return fn
   return hook
 
 
 def ask_for_data(prompt):
@@ skipping 2070 matching lines @@
   GenUsage(parser, 'help')
   return CMDhelp(parser, argv)
 
 
 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:]))