Improve "dcommit in git repo" error message.

git_auto_svn.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.
 
 """Performs all git-svn setup steps necessary for 'git svn dcommit' to work.
 
 Assumes that trunk of the svn remote maps to master of the git remote.
 
 Example:
@@ skipping 32 matching lines @@
   cmd = (SVN_EXE,) + cmd
   proc = subprocess2.Popen(cmd, **kwargs)
   ret, err = proc.communicate()
   retcode = proc.wait()
   if retcode != 0:
     raise subprocess2.CalledProcessError(retcode, cmd, os.getcwd(), ret, err)
 
   return ret, err
 
 
+def get_footer_svn_id(branch=None):
+  if not branch:
+    branch = root()
+  svn_id = None
+  message = run_git('log', '-1', '--format=%B', branch)
+  footers = parse_footers(message)
+  git_svn_id = get_unique(footers, 'git-svn-id')
+  match = GIT_SVN_ID_PATTERN.match(git_svn_id)
+  if match:
+    svn_id = match.group(1)
+  return svn_id
+
+
 def main(argv):
   # No command line flags. Just use the parser to prevent people from trying
   # to pass flags that don't do anything, and to provide 'usage'.
   parser = argparse.ArgumentParser(
       description='Automatically set up git-svn for a repo mirrored from svn.')
   parser.parse_args(argv)
 
   upstream = root()
-  message = run_git('log', '-1', '--format=%B', upstream)
-  footers = parse_footers(message)
-  git_svn_id = get_unique(footers, 'git-svn-id')
-  match = GIT_SVN_ID_PATTERN.match(git_svn_id)
-  assert match, 'No valid git-svn-id footer found on %s.' % upstream
-  print 'Found git-svn-id footer %s on %s' % (match.group(1), upstream)
+  svn_id = get_footer_svn_id(upstream)
+  assert svn_id, 'No valid git-svn-id footer found on %s.' % upstream
+  print 'Found git-svn-id footer %s on %s' % (svn_id, upstream)
 
-  parsed_svn = urlparse.urlparse(match.group(1))
+  parsed_svn = urlparse.urlparse(svn_id)
   path_components = parsed_svn.path.split('/')
   svn_repo = None
   svn_path = None
   for i in xrange(len(path_components)):
     try:
       maybe_repo = '%s://%s%s' % (
           parsed_svn.scheme, parsed_svn.netloc, '/'.join(path_components[:i+1]))
       print 'Checking ', maybe_repo
       run_svn('info', maybe_repo)
       svn_repo = maybe_repo
       svn_path = '/'.join(path_components[i+1:])
       break
     except subprocess2.CalledProcessError, e:
       if 'E170001' in str(e):
         print 'Authentication failed:'
         print e
         print ('Try running "svn ls %s" with the password'
                ' from https://chromium-access.appspot.com' % maybe_repo)
         print
       continue
-  assert svn_repo is not None, 'Unable to find svn repo for %s' % match.group(1)
+  assert svn_repo is not None, 'Unable to find svn repo for %s' % svn_id
   print 'Found upstream svn repo %s and path %s' % (svn_repo, svn_path)
 
   set_config('svn-remote.svn.url', svn_repo)
   set_config('svn-remote.svn.fetch',
              '%s:refs/remotes/%s' % (svn_path, upstream))
   print 'Configured metadata, running "git svn fetch". This may take some time.'
   for line in run_git_stream('svn', 'fetch').xreadlines():
     print line.strip()
   return 0
 
 
 if __name__ == '__main__':
   try:
     sys.exit(main(sys.argv[1:]))
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(1)