code: function readRef(gitFolder, refName) {
  	var ref
  	try {
  		ref = fs.readFileSync(path.join(gitFolder, refName), 'utf8').trim()
  		return isSHA1(ref) ? ref : ''
  	} catch (e) {
  		// Last chance: read from packed-refs
  		return readPackedRef(gitFolder, refName)
  	}
  }
docs: Return the commit name for the given ref name
  @param {string} gitFolder
  @param {string} refName
  @returns {string} - '' if not found