obsdiscovery: directly use the 'only' revset for obshashtree The revset now exists and is fast. This remove the needs to hold a reference to "revs" in that code.

obsdiscovery: extract push-obshashrange discovery in a function This will help us to simplify the code. The code is updated to directly use 'outgoing' missing so that is no longer needs to access 'futureheads'.