getBranch(branch, cb) {
      return this._request('GET', `/repos/${this.__fullname}/branches/${branch}`, null, cb);
   }
Get a single branch
@see https://developer.github.com/v3/repos/branches/#get-branch
@param {string} branch - the name of the branch to fetch
@param {Requestable.callback} cb - will receive the branch from the API
@returns {Promise} - the promise for the http request