Repository should have way to check whether repository is empty

[Jelmer Vernooij]

the Repository class should have a method to check whether it is empty. bzr-svn can significantly optimize fetching if it knows none of the revisions it has to fetch are present yet.

[Jelmer Vernooij (jelmer)]

Of course there is len(repository.revisions.keys()) but that can be *very* slow for some repository implementations (such as svn repositories) or unimplemented

[Jelmer Vernooij (jelmer)]

(and it scales with history)

[John A Meinel (jameinel)]

bzr could also make use of it with the smart server request (PendingAncestry grabs all of the ancestry of a given revision, without having to walk the history across the network).