format-patch: update append_signoff prototype This is a preparation step for merging with append_signoff from sequencer.c Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Brandon Casey <bcasey@nvidia.com> Reviewed-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>