revlog: rename constants (API) Feature flag constants don't need "NG" in the name because they will presumably apply to non-"NG" version revlogs. All feature flag constants should also share a similar naming convention to identify them as such. And, "RevlogNG" isn't a great internal name since it isn't obvious it maps to version 1 revlogs. Plus, "NG" (next generation) is only a good name as long as it is the latest version. Since we're talking about version 2, now is as good a time as any to move on from that naming.