Warn when git cl upload might be overwriting a newer patchset.

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 json
@@ skipping 1272 matching lines @@
   if not options.bypass_hooks:
     hook_results = cl.RunHook(committing=False,
                               may_prompt=not options.force,
                               verbose=options.verbose,
                               change=change)
     if not hook_results.should_continue():
       return 1
     if not options.reviewers and hook_results.reviewers:
       options.reviewers = hook_results.reviewers
 
+  if cl.GetIssue():
+    latest_patchset = cl.GetMostRecentPatchset(cl.GetIssue())
+    local_patchset = cl.GetPatchset()
+    if local_patchset != latest_patchset:
+      print ('The last upload made from this repository was patchset #%d but '
+            'the most recent patchset on the server is #%d.'
+            % (local_patchset, latest_patchset))
+      ask_for_data('About to upload; enter to confirm.')
+
   print_stats(options.similarity, options.find_copies, args)
   if settings.GetIsGerrit():
     return GerritUpload(options, args, cl)
   ret = RietveldUpload(options, args, cl)
   if not ret:
     git_set_branch_value('last-upload-hash', RunGit(['rev-parse', 'HEAD']))
 
   return ret
 
 
@@ skipping 589 matching lines @@
              'and retry or visit go/isgaeup.\n%s') % (e.code, str(e)))
 
   # Not a known command. Default to help.
   GenUsage(parser, 'help')
   return CMDhelp(parser, argv)
 
 
 if __name__ == '__main__':
   fix_encoding.fix_encoding()
   sys.exit(main(sys.argv[1:]))