Show old revision number in WebKit rolls.

tools/safely-roll-webkit.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.
 
 """Generate a CL to roll webkit to the specified revision number and post
 it to Rietveld so that the CL will land automatically if it passes the
 commit-queue's checks.
 """
 
@@ skipping 15 matching lines @@
 
 def process_deps(path, new_rev):
   """Update webkit_revision to |new_issue|.
 
   A bit hacky, could it be made better?
   """
   content = open(path).read()
   old_line = r'(\s+)"webkit_revision": "(\d+)",'
   new_line = r'\1"webkit_revision": "%d",' % new_rev
   new_content = re.sub(old_line, new_line, content, 1)
-  if new_content == content:
+  old_rev = re.search(old_line, content).group(2)
+  if not old_rev or new_content == content:
     die_with_error('Failed to update the DEPS file')
+
   open(path, 'w').write(new_content)
+  return old_rev
 
 
 def main():
   tool_dir = os.path.dirname(os.path.abspath(__file__))
   parser = optparse.OptionParser(usage='<new webkit rev>')
   parser.add_option('-v', '--verbose', action='count', default=0)
   options, args = parser.parse_args()
   logging.basicConfig(
       level=
           [logging.WARNING, logging.INFO, logging.DEBUG][
             min(2, options.verbose)])
   if len(args) != 1:
     parser.error('Need only one arg: new webkit revision to roll to.')
 
   root_dir = os.path.dirname(tool_dir)
   os.chdir(root_dir)
 
   new_rev = int(args[0])
-  msg = 'Roll webkit revision to %s' % new_rev
-  print msg
+  print 'Roll webkit revision to %s' % new_rev
 
   # Silence the editor.
   os.environ['EDITOR'] = 'true'
 
   old_branch = scm.GIT.GetBranch(root_dir)
   if old_branch == 'webkit_roll':
     parser.error(
         'Please delete the branch webkit_roll and move to a different branch')
   subprocess2.check_output(
       ['git', 'checkout', '-b', 'webkit_roll', 'origin/master'])
   try:
-    process_deps(os.path.join(root_dir, 'DEPS'), new_rev)
-    commit_msg = msg + '\n\nTBR=\n'
+    old_rev = process_deps(os.path.join(root_dir, 'DEPS'), new_rev)
+    commit_msg = 'Webkit roll %s:%s\n\nTBR=\n' % (old_rev, new_rev)
     subprocess2.check_output(['git', 'commit', '-m', commit_msg, 'DEPS'])
     subprocess2.check_call(['git', 'diff', 'origin/master'])
     subprocess2.check_call(['git', 'cl', 'upload', '--use-commit-queue'])
   finally:
     subprocess2.check_output(['git', 'checkout', old_branch])
     subprocess2.check_output(['git', 'branch', '-D', 'webkit_roll'])
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())