That looks extremely similar to http://www.notebooksapp.com. Is that the same app sold differently? I use notebooks app and am happy with it, always wanted a simple interface to my text/markdown files. But Quiver seems to have a few more options.

it's actually pretty close to what I need. Will try.

I currently use Quiver and save my notes to a google drive account. It gives me a similar workflow to what you're looking for, sans the ability to diff changes (unless I manually commit).