trailer: reorder format_trailers_from_commit() parameters

Currently there are two functions for formatting trailers in
<trailer.h>:

    void format_trailers(const struct process_trailer_options *,
                         struct list_head *trailers, FILE *outfile);

    void format_trailers_from_commit(struct strbuf *out, const char *msg,
                                     const struct process_trailer_options *opts);

and although they are similar enough (even taking the same
process_trailer_options struct pointer) they are used quite differently.
One might intuitively think that format_trailers_from_commit() builds on
top of format_trailers(), but this is not the case. Instead
format_trailers_from_commit() calls format_trailer_info() and
format_trailers() is never called in that codepath.

This is a preparatory refactor to help us deprecate format_trailers() in
favor of format_trailer_info() (at which point we can rename the latter
to the former). When the deprecation is complete, both
format_trailers_from_commit(), and the interpret-trailers builtin will
be able to call into the same helper function (instead of
format_trailers() and format_trailer_info(), respectively). Unifying the
formatters is desirable because it simplifies the API.

Reorder parameters for format_trailers_from_commit() to prefer

    const struct process_trailer_options *opts

as the first parameter, because these options are intimately tied to
formatting trailers. And take

    struct strbuf *out

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

Similarly, reorder parameters for format_trailer_info(), because later
on we will unify the two together.

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

ref-filter.c

@@ -1985,7 +1985,7 @@ static void grab_sub_body_contents(struct atom_value *val, int deref, struct exp
 			struct strbuf s = STRBUF_INIT;
 
 			/* Format the trailer info according to the trailer_opts given */
-			format_trailers_from_commit(&s, subpos, &atom->u.contents.trailer_opts);
+			format_trailers_from_commit(&atom->u.contents.trailer_opts, subpos, &s);
 
 			v->s = strbuf_detach(&s, NULL);
 		} else if (atom->u.contents.option == C_BARE)