Add a practracker mode that only regenerates over-broad exceptions

In legacy/trac#32213 (moved), I regenerated the practracker file, and then manually committed the over-broad exceptions.

It would be helpful if we had a --regen-overbroad option to practracker, that automatically regenerated the improvements we have made.

I feel like this should be easy, it's a combination of --regen and --list-overbroad. But I don't know the practracker design well enough to do it cleanly.