sequencer: simplify adding Signed-off-by: trailer

sequencer: simplify adding Signed-off-by: trailer

Add the Signed-off-by: trailer in one place rather than adding it to
the message when doing a recursive merge and specifying '--signoff'
when running 'git commit'. This means that if there are conflicts when
merging with a strategy other than 'recursive' the Signed-off-by:
trailer will be added if the user commits the resolution themselves
without passing '--signoff' to 'git commit'. It also simplifies the
in-process commit that is about to be added to the sequencer.

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