[jbergknoff]

It would be really helpful to go into some detail on the steps taken to clean up the git history.

[jerf]

It isn't what they did, but my recommendation for most such use cases is the BFG Repo-Cleaner: https://rtyley.github.io/bfg-repo-cleaner/

Don't be deceived by the "java" invocations, it's written in Scala, so it's HN-approved!

[albertzeyer]

Removing directories is simple.

http://git-scm.com/docs/git-filter-branch

But you can do almost anything via custom scripts.