sequencer: use the trailer iterator

sequencer: use the trailer iterator

Instead of calling "trailer_info_get()", which is a low-level function
in the trailers implementation (trailer.c), call
trailer_iterator_advance(), which was specifically designed for public
consumption in f0939a0eb1 (trailer: add interface for iterating over
commit trailers, 2020-09-27).

Avoiding "trailer_info_get()" means we don't have to worry about options
like "no_divider" (relevant for parsing trailers). We also don't have to
check for things like "info.trailer_start == info.trailer_end" to see
whether there were any trailers (instead we can just check to see
whether the iterator advanced at all).

Note how we have to use "iter.raw" in order to get the same behavior as
before when we iterated over the unparsed string array (char **trailers)
in trailer_info.

Signed-off-by: Linus Arver <linus@ucla.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>