An excellent tool I have used for similar purposes in the past (although it can do other things as well) is git-subtree. I recommend checking it out: https://github.com/apenwarr/git-subtree
Thanks! git-subtree looks very useful. Have you used it extensively? Any gotchas?
I haven't used it enough to really give you any gotchas. It has let me open source some stuff to github really easily though.
That was my thought as well, but then I thought that the requirements might be different here. git-subtree would allow you to create a git repository (with history) from a directory in a pre-existing repository. You could them just 'git rm -r' the directory from the original repository, but I'm not sure if the requirements here are to completely remove the history of that directory from the original repository too (because git-subtree doesn't do that, to my knowledge).
I agree, git-subtree doesn't quite do what they do in this blog post, but it covers similar ground and is really easy to use in comparison.