revset: fix generatorset race condition If two things were iterating over a generatorset at the same time, they could miss out on the things the other was generating, resulting in incomplete results. This fixes it by making it possible for two things to iterate at once, by always checking the _genlist at the beginning of each iteration. I was only able to repro it with pending changes from my other commits, but they aren't ready yet. So I'm unable to add a test for now.