Make it easier to do multiple bisect runs

For example the first run could skip everything that's not on hydra to quickly narrow down the range. The second run could then do the necessary rebuilds to pin down the specific commit.

One option would be to take control of the entire bisection process (instead of just writing a bisect script). Another would be to provide tooling for easily resetting `skip`s (like a `git bisect-edit`?).