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).