Make git auto-svn idempotent.

By directly using git-config rather than git-svn-init, this ensures that
crazy values don't get set. It could be "safer" (e.g. check to see if any
other git-svn configuration already exists and prompt before overwriting it),
but I think that simplicity is better here.

R=pgervais@chromium.org
BUG=425838

Review URL: https://codereview.chromium.org/675713002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@292640 0039d316-1c4b-4281-b951-d872f2087c98

git_auto_svn.py

@@ -84,7 +84,7 @@ def main(argv):
   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.'