worktree add -B: do the checkout test before update branch

worktree add -B: do the checkout test before update branch

If --force is not given but -B is, we should not proceed if the given
branch is already checked out elsewhere. add_worktree() has this test,
but it kicks in too late when "git branch --force" is already
executed. As a result, even though we correctly refuse to create a new
worktree, we have already updated the branch and mess up the other
checkout.

Repeat the die_if_checked_out() test again for this specific case before
"git branch" runs.

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