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.

stablesort: avoid attempting to sort a tuple This seems silly.

stablerange: use the filterparents utility

stablesort: use the filtered parents utility

depthcache: use parents filter in depth cache

parents: add a utility to filter parents There is various strange case of merge out there that needs to be handled. We add an utility function to handle them all.

stablerange: abstract the bit able to store cache into sql The part about actually storing the range is independent of the algorithm, so we should extract this before using it for the next iteration.

stablerange: split pure algorithm part from the on disk cache The stable cache file is getting long, we split the logic about keeping data on disk into a dedicated file.