That makes me think, has anyone tried to build a note taking app based on git? A tricky part for this kind of app is networking and synchronization of notes, and that's all taken care of by git. The "only" thing left would be to implement the mobile and desktop front ends.
I've done emacs org mode on git, and it works. I've thankfully never had to do a manual merge, I wonder what org mode thinks of git merge commentary inside the file. I imagine you'd have to drop out of org mode and into text mode to fix the merge.
I've also used a wiki as what amounts to little more than a note taking app, and it works. There are a couple wikis that can speak git as a backend store.
You might like to take a look at Irmin [1], which seems like it would make it easier to get what you're after. It's also part of a larger stack that will make distributed programming a lot easier when it lands [2].
Better yet -- CouchDB and PouchDB. You can use PouchDB for the browser side, CouchDB for the server, and Couchbase Mobile for the eventual mobile apps. Pros: offline and online; natural fit for document management; git-like revision and conflict management; simple to set up replication for backup. Cons: it's fallen out of favor; nobody seems to like writing the mapreduce code for searching and views.
