stablerange: use repo-carried stablesortcache That one is common to all and disk persisted

stablesort: expose the cache through the repository Let have a unique instance, keep it warm and accessible.

stablesort: implement an ondisk cache Persisting the cache on disk will be important for large repositories.

stablesort: realign a misaligned continue We should make a better use of the jump cache now.

stablesort: remove some dead code This closure is no longer in use.

stablesort: abstract all cache access This prepare an on disk version of the cache

stablerange: use first merge cache to skip over linear section If we are to skip over a large section of the range, skipping everything until the first merge seems like a good idea.

stablerange: add a new 'firstmerge' cache This cache store the first merge ancestors of a changesets. This is useful to avoid iteration over the changelog when building stablerange.

stablerange: drop unused `until` utility We no longer needs this function.

stablerange: use cached size data instead of walking the graph Less iteration is better.