'doesnotexist' wouldn't be a valid commit though, yes? From @zedshaw, when I asked him: '@edropple Yep, you can just push randomness in various ways, my favorite is: git push -f origin master:BIGASSHEX'
That doesn't actually do anything though. It pushes your local master branch to a remote branch called BIGASSHEX (creating it if it doesn't exist). So if you generate a new bigass hex each time you'll end up with a ton of randomly-named branches that all point to the same commit, so they use no space.
I think he found a way to take down Github (having to do with having too many branches in a project), but not actually a hole in Git.