code: function _getSHA(id, callback) {
          repo.contents("").fetch({
              ref: options.branch
          }, function(err, res) {
              if (err) return callback(err);
              var sha = res.reduce(function(previousValue, item) {
                  return item.name === id ? item.sha : previousValue;
              }, "");
              callback(err, sha);
          });
      }
docs: get the SHA corresponding to id at the HEAD of options.branch
  
  @access private
  @param {String} id
  @param {Function} callback called with (err, result)