remote-bzr: convert all unicode keys to str

Otherwise some versions of bazaar might barf.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
remote-bzr: avoid bad refs

Turns out fast-export throws bad 'reset' commands because of a behavior
in transport-helper that is not even needed.

We should ignore them, otherwise we will threat them as branches and
fail.

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