code: func githubHEAD(gerritHEAD string) (string, error) {
  	const (
  		headAPI  = "https://api.github.com/repos/perkeep/perkeep/commits/HEAD"
  		mimeType = "application/vnd.github.VERSION.sha"
  	)
  	req, err := http.NewRequest("GET", headAPI, nil)
  	if err != nil {
  		return "", err
  	}
  	req.Header.Add("If-None-Match", `"`+gerritHEAD+`"`)
  	req.Header.Add("Accept", mimeType)
  	resp, err := http.DefaultClient.Do(req)
  	if err != nil {
  		return "", err
  	}
  	defer resp.Body.Close()
  	if resp.StatusCode == http.StatusNotModified {
  		return gerritHEAD, nil
  	}
  	ghCommit, err := ioutil.ReadAll(resp.Body)
  	if err != nil {
  		return "", err
  	}
  	return string(ghCommit), nil
  }
docs: // githubHEAD returns the hash of the HEAD commit on the github repo.
  // The gerritHEAD argument is used as an optimization in the request to github:
  // if it is found as the HEAD commit, the request is not counted in our
  // non-authenticated requests quota.