getRef(ref, cb) {
      return this._request('GET', `/repos/${this.__fullname}/git/refs/${ref}`, null, cb);
   }
Get a reference
@see https://developer.github.com/v3/git/refs/#get-a-reference
@param {string} ref - the reference to get
@param {Requestable.callback} [cb] - will receive the reference's refSpec or a list of refSpecs that match `ref`
@return {Promise} - the promise for the http request