revset: avoid implicit None testing in revset Implicit None testing is a very good way to get in trouble. We explicitly test for None.

revset: speed up existence checks for ordered filtered sets Previously, calling 'if foo:' on a ordered filtered set would start iterating in whatever the current direction was and return if a value was available. If the current direction was ascending, but the set had a fastdesc available, this meant we did a lot more work than necessary. If this was applied without my previous max/min fixes, it would improve max() performance (this was my first attempt at fixing the issue). Since those previous fixes went in though, this doesn't have a visible benefit in the benchmarks, but it does seem clearly better than it was before so I think it should still go in.