I built Octotree a while back [1]. It's an open-source browser extension (Chrome, Firefox, Opera and Safari) that adds IDE-like tree view to GitHub. Due to popular request, GitLab is now also supported. Hopes you will find it useful. Link to install: https://github.com/buunguyen/octotree#install-on-chrome-firefox-and-opera.
[1]: https://news.ycombinator.com/item?id=7740226