util: avoid mutable default arguments I almost introduced a bug around this code by accidentally mutating a default argument. There's no reason for these to exist.

obsstore: fast path to check if obsstore is empty

obsstore: delay loading markers from obsstore file This will allow us to use cached revisions without parsing obsstore. Because _version isn't determined at __init__, the debugobsconvert command no longer works correctly. I'll fix it by a separate patch for the evolve extension.