format_trailer_info(): move "fast path" to caller

format_trailer_info(): move "fast path" to caller

This is another preparatory refactor to unify the trailer formatters.

This allows us to drop the "msg" parameter from format_trailer_info(),
so that it take 3 parameters, similar to format_trailers() which also
takes 3 parameters:

    void format_trailers(const struct process_trailer_options *opts,
                         struct list_head *trailers,
                         struct strbuf *out)

The short-term goal is to make format_trailer_info() be smart enough to
deprecate format_trailers(). And then ultimately we will rename
format_trailer_info() to format_trailers().

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