format_trailer_info(): use trailer_item objects

format_trailer_info(): use trailer_item objects

This is another preparatory refactor to unify the trailer formatters.

Make format_trailer_info() operate on trailer_item objects, not the raw
string array.

We will continue to make improvements, culminating in the renaming of
format_trailer_info() to format_trailers(), at which point the
unification of these formatters will be complete.

In order to avoid breaking t4205 and t6300, flip *_success to *_failure
in the affected test cases. Add a temporary
"test_trailer_option_expect_failure" wrapper which we will use along
with "test_expect_failure" in the next commit to avoid breaking tests.
When the dust settles with the refactors a few more commits later, we
will drop the use of *_failure to make the tests truly pass again.

When the preparatory refactors are complete,
we'll be able to drop the use of *_failure that we introduce here.

Signed-off-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>