Fix "git cl diff" when run on branch without an issue.

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."""
 
 from distutils.version import LooseVersion
@@ skipping 2142 matching lines @@
   cl = Changelist()
   # Ensure there actually is an issue to close.
   cl.GetDescription()
   cl.CloseIssue()
   return 0
 
 
 def CMDdiff(parser, args):
   """shows differences between local tree and last upload."""
   cl = Changelist()
+  issue = cl.GetIssue()
   branch = cl.GetBranch()
+  if not issue:
+    DieWithError('No issue found for current branch (%s)' % branch)
   TMP_BRANCH = 'git-cl-diff'
   base_branch = RunGit(['merge-base', cl.GetUpstreamBranch(), 'HEAD']).strip()
 
   # Create a new branch based on the merge-base
   RunGit(['checkout', '-q', '-b', TMP_BRANCH, base_branch])
   try:
     # Patch in the latest changes from rietveld.
-    rtn = PatchIssue(cl.GetIssue(), False, False, None)
+    rtn = PatchIssue(issue, False, False, None)
     if rtn != 0:
       return rtn
 
     # Switch back to starting brand and diff against the temporary
     # branch containing the latest rietveld patch.
     subprocess2.check_call(['git', 'diff', TMP_BRANCH, branch])
   finally:
     RunGit(['checkout', '-q', branch])
     RunGit(['branch', '-D', TMP_BRANCH])
 
@@ skipping 136 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:]))