mergestate._resolve: store return code and action for each file We're going to need this to compute (a) updated/merged/unresolved counts, and (b) actions to perform on the dirstate.

revsetbenchmarks: support benchmarking changectx loading Many revset consumers construct changectx instances for each returned result. Add support for benchmarking this to our revset benchmark script. In the future, we might want to have some kind of special syntax in the parsed revset files to engage this mode automatically. This would enable us to load changectxs for revsets that do that in the code and would more accurately benchmark what's actually happening. For now, running all revsets with or without changectxs is sufficient.

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.