wt-status.c: split bisect detection out of wt_status_get_state()

And make it work with any given worktree, in preparation for (again)
find_shared_symref(). read_and_strip_branch() is deleted because it's
no longer used.

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