obsdiscovery: extract '_canobshashtree' having this as a simple function will help use to generalise the obsdiscovery algorithm we use.

obsdiscovery: move code inserting discovery at the end of the file This makes it easier to reuser function defined earlier in the file.

obsdiscovery: trust core to do the right thing if no discovery is available This is fixed for a long time now.

obsdiscovery: factor the "do we push markers" check out This clarify the code a bit.

stablerange: also respect the 'warm-cache' config This will help large install to enable obshashrange as "opt-in" without impacting other repositories.

debugstablerange: cleanly "Abort" when no revision are specified Previously we had a traceback from a max on an empty sequence.

stablerange: use last recently used caching for revisions associated to ranges This does not make things faster but at least it make them able to finish on large repositories.