Migrate a directory (with history) to a separate repo.

migrate-subdirectory

# First create the blank repository, then...
git clone https://github.com/borkweb/whatever.git tmp-repo
git clone --no-hardlinks tmp-repo new-repo
cd new-repo
git filter-branch --subdirectory-filter directory-to-migrate HEAD -- --all
git reset --hard
git gc --aggressive
git prune
git remote rm origin
git remote add origin https://github.com/borkweb/new-repo.git
git push origin master --force

Why not use "git subtree split"?

http://makingsoftware.wordpress.com/2013/02/16/using-git-subtrees-for-repository-separation/