remote-bzr: simplify get_remote_branch()

No need for 'origin', it's only needed for the bzrdir 'sprout' method,
which can be greatly simplified.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
remote-bzr: delay cloning/pulling

Until the branch is actually going to be used.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
remote-bzr: change global repo

It's not used anyway.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
remote-bzr: trivial cleanups

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
remote-bzr: reorganize the way 'wanted' works

If the user specified a list of branches, we ignore what the remote
repository lists, and simply use the branches directly. Since some
remotes don't report the branches correctly, this is useful.

Otherwise either fetch the repo, or the branch.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>