Describe the feature

The "request changes" review causes problems if there are checks that might take a while to know the answer to (for example, other runs).

For full safety we should "request changes" while the checks are still running, so the PR isn't accidentally merged before the check turns green.

But then a merge-from-live → checks in progress → request changes → Mergify dequeue, which is not what we want.

Instead, a status has 3 possible values: "good / bad / don't know yet", which we should use for this purpose.