createTree(tree, baseSHA, cb) {
      return this._request('POST', `/repos/${this.__fullname}/git/trees`, {
         tree,
         base_tree: baseSHA, // eslint-disable-line camelcase
      }, cb);
   }
Create a new tree in git
@see https://developer.github.com/v3/git/trees/#create-a-tree
@param {Object} tree - the tree to create
@param {string} baseSHA - the root sha of the tree
@param {Requestable.callback} cb - will receive the new tree that is created
@return {Promise} - the promise for the http request