config: add hasconfig method and supporting plumbing We add the hasconfig method to make it possible to distinguish between a config value that was never supplied and one that is empty.

verify: replace "output parameters" by return values _verifychangelog() and _verifymanifest() accept dictionaries that they populate. We pass in empty dictionaries, so it's clearer to create them in the functions and return them.

shelve: hook afterresolvedstates