code: func (r *Repo) checkoutCommit(commitHash string) error {
  	var err error
  	params := []string{"checkout", commitHash}
  	if err = r.gitCmd(params, r.Path); err == nil {
  		Logger().Printf("Commit %v checkout done.\n", commitHash)
  	}
  	return err
  }
docs: // checkoutCommit checks out the specified commitHash.