stablerange: add an assert to detect buggy range Nothing can be negative in there, we add an assert to make sure it is so.

stablerange: warn cache for all relevant ancestors range Warming cache in the right order will prevent recursion to happens and unlock usage on large repository.

stablesort: move parent range computation into its own method Now that we have running code, we'll need to avoid too deep recursion. We extract the piece dealing with the parent range before we write more code to warm it.

stablerange: drop the basic inheritance from the 'mergepoint' version All necessary bits are implemented now (except for warmup which we explicitly bypass for now).

stablerange: cache known subrange at the stablerangecached level That way, the 'stablerange_mergepoint' class also benefit from it.

stablerange: use sort cache to build 'mergepoint' information Using the walk cache gives use a better complexity we need to scale. There are further improvement we could make, but this is a good first step.

stablesort: make the iteration from head available to all This will be useful for the stablerange code to work properly.

depth: fix compare mode for `debugdepth` The compare mode was broken for silly reasons.