[MERGE] Move use_existing and create_prefix all the way down to clone_on_transport, reducing duplicate work.

[Andrew Bennetts]

Robert Collins wrote:
> This isn't a dramatic change, but it does cut one round trip off
> creating new branches - we were creating the directory and then trying
> again, just in case it hadn't worked.

bb:approve

Looks reasonable to me.  I'm a bit ambivalent about raising BzrCommandError
from non-UI code, but I don't think that's a particularly important issue
here.  Otherwise pushing this logic down looks good.

-Andrew.