sequencer.c: always separate "(cherry picked from" from commit body
Start treating the "(cherry picked from" line added by cherry-pick -x
the same way that the s-o-b lines are treated. Namely, separate them
from the main commit message body with an empty line.
Introduce tests to test this functionality.
Signed-off-by: Brandon Casey <bcasey@nvidia.com> Reviewed-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>