I've had good results from using `git filter-branch --index-filter` to keep everything related to the exported package and remove everything else. This keeps co

I've had good results from using `git filter-branch --index-filter` to keep everything related to the exported package and remove everything else. This keeps commit history in such a way that `git log --follow` still shows older commits in the new repo, which is nice. I could never figure out how to make that work with `git subtree`.