code: func (s *GitService) GetTree(ctx context.Context, owner string, repo string, sha string, recursive bool) (*Tree, *Response, error) {
  	u := fmt.Sprintf("repos/%v/%v/git/trees/%v", owner, repo, sha)
  	if recursive {
  		u += "?recursive=1"
  	}
  
  	req, err := s.client.NewRequest("GET", u, nil)
  	if err != nil {
  		return nil, nil, err
  	}
  
  	t := new(Tree)
  	resp, err := s.client.Do(ctx, req, t)
  	if err != nil {
  		return nil, resp, err
  	}
  
  	return t, resp, nil
  }
docs: // GetTree fetches the Tree object for a given sha hash from a repository.
  //
  // GitHub API docs: https://developer.github.com/v3/git/trees/#get-a-tree