Amend the freeing logic added in e6e045f8031 (diff.c: buffer all
output if asked to, 2017-06-29) to free the containing "buf" in
addition to its members.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>