trailer_info_get(): reorder parameters

trailer_info_get(): reorder parameters

This is another preparatory refactor to unify the trailer formatters.

Take

    const struct process_trailer_options *opts

as the first parameter, because these options are required for
parsing trailers (e.g., whether to treat "---" as the end of the log
message). And take

    struct trailer_info *info

last, because it's an "out parameter" (something that the caller wants
to use as the output of this function).

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