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.

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.