revset: fix generatorset race condition...
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.
