Improve "dcommit in git repo" error message.

git_footers.py

 #!/usr/bin/env python
 # Copyright 2014 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.
 
 import argparse
 import re
 import sys
 
 from collections import defaultdict
@@ skipping 30 matching lines @@
   if not all(footers):
     return defaultdict(list)
 
   footer_map = defaultdict(list)
   for (k, v) in footers:
     footer_map[normalize_name(k)].append(v.strip())
 
   return footer_map
 
 
+def get_footer_svn_id(branch=None):
+  if not branch:
+    branch = git.root()
+  svn_id = None
+  message = git.run('log', '-1', '--format=%B', branch)
+  footers = parse_footers(message)
+  git_svn_id = get_unique(footers, 'git-svn-id')
+  if git_svn_id:
+    match = GIT_SVN_ID_PATTERN.match(git_svn_id)
+    if match:
+      svn_id = match.group(1)
+  return svn_id
+
+
 def get_unique(footers, key):
   key = normalize_name(key)
   values = footers[key]
   assert len(values) <= 1, 'Multiple %s footers' % key
   if values:
     return values[0]
   else:
     return None
 
 
@@ skipping 80 matching lines @@
         print '%s: %s' % (k, v)
   return 0
 
 
 if __name__ == '__main__':
   try:
     sys.exit(main(sys.argv[1:]))
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(1)