am: fix signoff when other trailers are present

am: fix signoff when other trailers are present

If there was no 'Signed-off-by:' trailer but another trailer such as
'Reported-by:' then 'git am --signoff' would add a blank line between
the existing trailers and the added 'Signed-off-by:' line. e.g.

    Rebase accepts '--rerere-autoupdate' as an option but only honors
    it if '-m' is also given. Fix it for a non-interactive rebase by
    passing on the option to 'git am' and 'git cherry-pick'.

    Reported-by: Junio C Hamano <gitster@pobox.com>
    Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Fix by using the code provided for this purpose in sequencer.c.
Change the tests so that they check the formatting of the
'Signed-off-by:' lines rather than just grepping for them.

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