revsfromcache: update cache for the top slice if possible Same logic, we update the cache if have the data.

revsfromrange: skip setting the cache for length-1 top entry The content of the range is trivial to compute.

revsfromrange: update cache for parentrange directly in the code We update it where it matters if we detect that we have the data.

revsfromramge: hard code the single changeset range case That case is trivial and triggering and full stable sort for it seems a bit silly.

stablerange: introduce caching for the full revision in a set Such cache proved handy in the "per-range" class so we carry it along to the unified class. cf documentation for details.

stablerange: add a cache for stablesort ordering This will be very handy for merge, cf inline documentation for details.

stablerange: move revs computation within the main class We still need to compute the revision withing a range when we slice a merge. This is the last large logic that remains in the individual class and we migrate is on the main class.