It's possible that the previous tip goes away, we should not assume it's
always present. Fortunately we are only using it to calculate the
progress to display to the user, so only that needs to be fixed.
Also, add a test that triggers this issue.
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>