RemoteBranch.revision_id_to_revno uses VFS, but doesn't need to.

[Andrew Bennetts]

RemoteBranch.revision_id_to_revno is a simple _ensure_real shim at the moment, but it doesn't need to be.

We already have revision_history and get_parent_map RPCs. Also, RemoteBranch already inherits from Branch, so the implementation of either Branch.revision_id_to_revno or BzrBranch8.revision_id_to_revno both work fine.

(The latter would also require replacing RemoteRepository.iter_reverse_revision_history with a non-_ensure-real version, again just using Repository's implementation directly would work fine.)