getTree(treeSHA, cb) {
      return this._request('GET', `/repos/${this.__fullname}/git/trees/${treeSHA}`, null, cb);
   }
Get a description of a git tree
@see https://developer.github.com/v3/git/trees/#get-a-tree
@param {string} treeSHA - the SHA of the tree to fetch
@param {Requestable.callback} cb - will receive the callback data
@return {Promise} - the promise for the http request