stablesort: allow a callback to be triggered on merge Storing some information as we sort is going to be useful to avoid performing multiple sort while computing the stablerange.

minor simplification around rangelength

more explicite name in revsfromrange

stablerange: cache parents We happens to be doing and awful amount of parent call. We cache them locally for efficiency.

merge-slicing: avoid doing the same work twice We have already computed the list of revision in that bottom slice as 'hrevs' so we do not need to compute it a second time.

stablerange: fix a bug when a top slice ended on a merge Our "smart" detection of merge was buggy if the top slice ended on a merge. This is not fixed and tested.

slicesrangeat: stop double setting the revsinranges cache The cache should have already been filled by the logic warming the cache for the parent.