message: Fetch only master branch of the registry index
  Should close
diff: @@ -97,7 +97,7 @@ impl<'cfg> RegistryData for RemoteRegistry<'cfg> {
  // git fetch origin
  let url = self.source_id.url().to_string();
  - let refspec = "refs/heads/*:refs/remotes/origin/*";
  + let refspec = "refs/heads/master:refs/remotes/origin/master";
  git::fetch(&repo, &url, refspec, self.config).chain_error(|| {
  human(format!("failed to fetch `{}`", url))
  