It would be cool to make a tool for 'playing back' git repos like this. I could see it having media player style controls so you can jump anywhere in progress, step to the next or previous commit, etc. At each commit, you could have a rendering of the diff, along with interspersed annotations. Maybe there could be a format for annotations, so multiple people could submit annotations for the same repos (or subsets of repos), and you'd end up with different tutorials.

So, in order to make one of these tutorials, you feed the software the url to the repo, start/end commit hashes, and some JSON or whatever that contains your annotations—and it gives you back an interactive presentation of annotated diffs.