sequencer: fix cleanup with --signoff and -x

sequencer: fix cleanup with --signoff and -x

Before commit 356ee4659b ("sequencer: try to commit without forking 'git
commit'", 2017-11-24) when --signoff or -x were given on the command
line the commit message was cleaned up with --cleanup=space or
commit.cleanup if it was set. Unfortunately this behavior was lost when
I implemented committing without forking. Fix this and add some tests to
catch future regressions.

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