No need to use git, since you can just use diff(1) to find the unfollowers between any two dates. I've been doing this for a while with a Go program I created[0].

Unfortunately, people who change their Twitter handles (which I found surprisingly common) show up as false positives - you either need to filter these out manually or match against the user id[1].

[0] https://github.com/ChimeraCoder/twitter-follower-logger

[1] Adding that as a separate field and then calling `cut -f2` would be a quick hack, now that I think about it.