worktree.c: check whether branch is bisected in another worktree

worktree.c: check whether branch is bisected in another worktree

Similar to the rebase case, we want to detect if "HEAD" in some worktree
is being bisected because

1) we do not want to checkout this branch in another worktree, after
   bisect is done it will want to go back to this branch

2) we do not want to delete the branch is either or git bisect will
   fail to return to the (long gone) branch

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>