Since `git worktree add` uses `git checkout` when `[<branch>]` is used,
and `git checkout -` is already supported, it makes sense to allow the
same shortcut in `git worktree add`.
Signed-off-by: Jordan DE GEA <jordan.de-gea@grenoble-inp.org> Signed-off-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr> Signed-off-by: Junio C Hamano <gitster@pobox.com>