add -p: fix checkout -p with pathological context

add -p: fix checkout -p with pathological context

Commit fecc6f3a68 ("add -p: adjust offsets of subsequent hunks when one is
skipped", 2018-03-01) fixed adding hunks in the correct place when a
previous hunk has been skipped. However it did not address patches that
are applied in reverse. In that case we need to adjust the pre-image
offset so that when apply reverses the patch the post-image offset is
adjusted correctly. We subtract rather than add the delta as the patch
is reversed (the easiest way to think about it is to consider a hunk of
deletions that is skipped - in that case we want to reduce offset so we
need to subtract).

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>