branching from hpss doesn't preserve repository format

In bzr 0.92, the remote repository always reports that it's a knit repository (??). bzr branch, when creating a new repository, normally uses the same format as the origin, but when coming from an hpss repository it creates a new knit repository.

Aside from being inconsistent with the normal behavior of the command, this means that we do format conversion in flight, and so branching is much slower than it would normally be.

This can be worked around by creating a destination repository before branching.