Another thing you can do when you have two versions, one which fails and one which doesn't, but where the commits are of the messy work in progress nature and not atomic changes, is gradually stage the changes from the bad version onto the good version until it starts failing
That's a smart way to do it. When I was in this situation without having thought of that, I ended up chunking the monster commit into as small changes as possible with `git add -p`, and then used git rebase as my bisect (since I didn't know how bisect worked either) by rewinding through time until i had found the micro-commit that contained the error.
Basically git checkout -p until things fail