fix 'listCommits' when only callback is given

Author: WJXHenry

lib/Repository.js

@@ -187,8 +187,10 @@ class Repository extends Requestable {
     * @return {Promise} - the promise for the http request
     */
    listCommits(options, cb) {
-      options = options || {};
-
+      if (typeof options === 'function') {
+         cb = options;
+         options = {};
+      }
       options.since = this._dateToISO(options.since);
       options.until = this._dateToISO(options.until);