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.