checkout: check for "Previous HEAD" notice in t2020
If we leave a detached head, exactly one of two things happens: either checkout warns about it being an orphan or describes it as a courtesy. Test t2020 already checked that the warning is shown as needed. This patch also checks for the description.
Signed-off-by: Rene Scharfe <email@example.com> Signed-off-by: Junio C Hamano <firstname.lastname@example.org>