1. Add a new commit undoing the whitespace stuff
2. Run `git rebase -i HEAD^^`  An editor will come up asking what you want to do with each commit.  Mark the last commit (in which you removed the whitespace) as "squash" to merge it into the previous commit (effectively undoing the whitespace)
3. Run `git push origin breakit -f` to force-push your newly rebased branch to GitHub; this will automatically update the PR