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.

subranges: remove the recursivity of the call to isubranges(parentrange) We add some logic to ensure we'll have hot cache for the parent ranges when that matters, the cache is filled from ancestors to descendant to ensure this. The range are still 'created from descendant to ancestors to fill the revsfromrange cache since it important.

subranges: detach cache logic from computation logic Having both the logic around cache checking and setting makes is a bit harder to follow. In addition, this allow to gather the computation logic next to the other related function.

findmissingrange: properly queue new subrange for slicing The previous code was buggy and used the wrong variable leading to no extra slicing being performed to file the sample at the requested size.

findmissingrange: fix reversed value in debug output "oops"