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>

trailer.h

@@ -101,8 +101,9 @@ void trailer_info_release(struct trailer_info *info);
  *     only the trailer block itself, even if the "only_trailers" option is not
  *     set.
  */
-void format_trailers_from_commit(struct strbuf *out, const char *msg,
-				 const struct process_trailer_options *opts);
+void format_trailers_from_commit(const struct process_trailer_options *opts,
+				 const char *msg,
+				 struct strbuf *out);
 
 /*
  * An interface for iterating over the trailers found in a particular commit