perf: support obtaining contexts from perfrevset Previously, perfrevset called repo.revs(), which only returns integer revisions. Many revset consumers call repo.set(), which returns changectx instances. Or they obtain a context manually later. Since obtaining changectx instances when evaluating revsets is common, this patch adds support for benchmarking this use case. While we added an if conditional for every benchmark loop, it doesn't appear to matter since revset evaluation dwarfs the cost of a single if.