filectx: if we have a _descendantrev, use it to adjust linkrev This lets us use _adjustlinkrev lazily.

copies: use linkrev for file tracing limit This lets us lazily evaluate _adjustlinkrev.

filectx: use linkrev to sort ancestors We're going to make rev() lazily do _adjustlinkrevs, and we don't want that to happen when we're quickly tracing through file ancestry without caring about revs (as we do when finding copies). This takes us back to pre-linkrev-correction behavior, but shouldn't regress us relative to the last stable release.