See https://help.github.com/articles/splitting-a-subfolder-out-into-a-new-repository/

This can be done by:

1. Clone ATC, prior to the auth removal.
2. See above post to create a local branch containing all commits for the auth subdirectory.
3. Clone current skymarshal repo and somehow apply the (few) commits we have here since the split. Cherry-pick? Manual patch applying?