stablesort: rework jump gathering The old code was buggy in some case, the new code give the same result as the 'headstart' version.

stablesort: stop recording jump type There are no mature user of this "jump type" data and the current implementation is known to be buggy. So we drop the logic for now. This will make on disk storage simpler. The data will be reintroduced later

stablesort: pass a jump recording function instead of a list This seems cleaner to abstract the underlying storage to the lower level. This also makes use more flexible about jump detection.

stablesort: minor indent fix

stablesort: clarify subcall to the exclusive side Before doing deeper rework of this logic, we tackle the simplest parts.

docgraph: update test output with new output

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.