branch: show rebase/bisect info when possible instead of "(no branch)"

This prints more helpful info when HEAD is detached: is it detached
because of bisect or rebase? What is the original branch name in those
cases?

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>