remote-bzr: convert all unicode keys to str

Otherwise some versions of bazaar might barf.

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

Versions of fast-export before v1.8.2 throws a bad 'reset' commands
because of a behavior in transport-helper that is not even needed.
We should ignore them, otherwise we will treat them as branches and
fail.

This was fixed in v1.8.2, but some people use this script in older
versions of git.

Also, check if the ref was a tag, and skip it for now.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>