Make git auto-svn idempotent.

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 66 matching lines @@
       run_svn('info', maybe_repo)
       svn_repo = maybe_repo
       svn_path = '/'.join(path_components[i+1:])
       break
     except subprocess2.CalledProcessError:
       continue
   assert svn_repo is not None, 'Unable to find svn repo for %s' % match.group(1)
   print 'Found upstream svn repo %s and path %s' % (svn_repo, svn_path)
 
   prefix = upstream.rsplit('/')[0]
-  run_git('svn', 'init', '--prefix=%s' % prefix, '-T', svn_path, svn_repo)
+  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()
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))