sequencer.c: teach append_signoff to avoid adding a duplicate newline

sequencer.c: teach append_signoff to avoid adding a duplicate newline

Teach append_signoff to detect whether a blank line exists at the position
that the signed-off-by line will be added, and refrain from adding an
additional one if one already exists.  Or, add an additional line if one
is needed to make sure the new footer is separated from the message body
by a blank line.

Signed-off-by: Brandon Casey <bcasey@nvidia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>