worktree: add: introduce --checkout option

worktree: add: introduce --checkout option

By adding this option which defaults to true, we can use the
corresponding --no-checkout to make some customizations before
the checkout, like sparse checkout, etc.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Ray Zhang <zhanglei002@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>