sequencer: use trailer's trailer layout

sequencer: use trailer's trailer layout

Make sequencer use trailer.c's trailer layout definition, as opposed to
parsing the footer by itself. This makes "commit -s", "cherry-pick -x",
and "format-patch --signoff" consistent with trailer, allowing
non-trailer lines and multiple-line trailers in trailer blocks under
certain conditions, and therefore suppressing the extra newline in those
cases.

Consistency with trailer extends to respecting trailer configs.  Tests
have been included to show that.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>