stablerange: move the subrange cache into our new class This is the first step toward having more logic in that class

depth: extract code dedicated to depth of a merge in its own function The merge case is more complicated than the regular one, we extract is for the sake of clarity.

stablerange: move 'depth' inside a new 'stablerange' module The stablerange used for discovery requires significant amount of code. There is algorithme, cache, cache persistence, etc. So we create a dedicated module for it. The function is directly moved into a rich class handling cache (for now in memory) because we know we will need it.

obsdiscovery: document the status of the module I figured i would be useful to document what we expect from the code in each module.

split: move the debugcommand into a dedicated module The code related to debugobsstorestat is fairly independant, we move it into its own module.

legacy: move 'debugrecordpruneparents' in the extensions The transition is a couple of year old now, repository with the old format must be quite hard to find by now.

checks: correct the shebang line filtering for python files As it is, the only related file is docs/test2rst.py, which was covered by **.py. Not sure if it matters, but most patterns in core tests are for "#!.*?python". (Though there are a couple "#!.*python" tests.)