I must be missing something here.  Using e.g. Mercurial, I would start my refactoring and commit often.  If a test fails at the end and I can't see why then just update to the version before the refactoring (assuming I hadn't ever ran tests in the mean time), run the test, if it passes update to the next revision and so on.  I would expect this to take seconds or minutes to isolate.  What kind of revisions system is he using that this even comes up?