Comment by rav

Given a commit that both refactors (A) and adds a feature (B), you can go into the codebase and remove the new feature by hand (B^-1), commit the feature removal, and immediately revert the feature removal. This leads to three commits: (A B), B^-1, and B. Squash the first two commits to obtain a commit that only refactors, and another commit that only adds the new feature. I've written more about this technique ("the Hammer") here: https://github.com/Mortal/gittalk