code: func NewCheckout(commits notifypb.GitilesCommits) Checkout {
  	results := make(Checkout, len(commits.GetCommits()))
  	for _, gitilesCommit := range commits.GetCommits() {
  		results[protoutil.GitilesRepoURL(gitilesCommit)] = gitilesCommit.Id
  	}
  	return results
  }
docs: // NewCheckout creates a new Checkout populated with the repositories and revision
  // found in the GitilesCommits object.