Fix going back to root directory with history in notebook list

Bug found during pair programming with @mpacer

Author: takluyver

notebook/static/tree/js/notebooklist.js

@@ -102,6 +102,7 @@ define([
         this.sessions = {};
         this.base_url = options.base_url || utils.get_body_data("baseUrl");
         this.notebook_path = options.notebook_path || utils.get_body_data("notebookPath");
+        this.initial_notebook_path = this.notebook_path;
         this.contents = options.contents;
         if (this.session_list && this.session_list.events) {
             this.session_list.events.on('sessions_loaded.Dashboard',
@@ -358,7 +359,8 @@ define([
         var that = this;
         // Add an event handler browser back and forward events
         window.onpopstate = function(e) {
-            var path = window.history.state ? window.history.state.path : '';
+            var path = (window.history.state && window.history.state.path) ?
+                    window.history.state.path : that.initial_notebook_path;
             that.update_location(path);
         };
         var breadcrumb = $('.breadcrumb');