code: func (s *RepositoriesService) CompareCommits(ctx context.Context, owner, repo string, base, head string) (*CommitsComparison, *Response, error) {
  	u := fmt.Sprintf("repos/%v/%v/compare/%v...%v", owner, repo, base, head)
  
  	req, err := s.client.NewRequest("GET", u, nil)
  	if err != nil {
  		return nil, nil, err
  	}
  
  	comp := new(CommitsComparison)
  	resp, err := s.client.Do(ctx, req, comp)
  	if err != nil {
  		return nil, resp, err
  	}
  
  	return comp, resp, nil
  }
docs: // CompareCommits compares a range of commits with each other.
  // todo: support media formats - https://github.com/google/go-github/issues/6
  //
  // GitHub API docs: https://developer.github.com/v3/repos/commits/#compare-two-commits