stablesort: record, cache and reuse jump Iterating below a merge means two things: 1) iterate over the part exclusive to the higher parents, 2) iterate from the lower parents. While iterating on the exclusive part, there will be case were we just go the next natural parent, and case were we'll have to "jump" to another revision. If we record all point this "jump" happens and their target, we can easily reproduce the iteration in the future. With that information we can iterate over the exclusive part of the merge without having to compute it entirely. In addition we store the reason of the jump. This will help the stable range processing later.

stablesort: fix head start computation The recursion was wrong and provided wrong result. As a side effect, this seems to hint that `stablesort_mergepoint_bounded` is not giving the right result for complex set.