format_trailers_from_commit(): indirectly call trailer_info_get()

This is another preparatory refactor to unify the trailer formatters.

For background, note that the "trailers" string array is the
`char **trailers` member in `struct trailer_info` and that the
trailer_item objects are the elements of the `struct list_head *head`
linked list.

Currently trailer_info_get() only populates `char **trailers`. And
parse_trailers() first calls trailer_info_get() so that it can use the
`char **trailers` to populate a list of `struct trailer_item` objects

Instead of calling trailer_info_get() directly from
format_trailers_from_commit(), make it call parse_trailers() instead
because parse_trailers() already calls trailer_info_get().

This change is a NOP because format_trailer_info() (which
format_trailers_from_commit() wraps around) only looks at the "trailers"
string array, not the trailer_item objects which parse_trailers()
populates. For now we do need to create a dummy

    LIST_HEAD(trailer_objects);

because parse_trailers() expects it in its signature.

In a future patch, we'll change format_trailer_info() to use the parsed
trailer_item objects (trailer_objects) instead of the `char **trailers`
array.

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

trailer.c

@@ -1140,9 +1140,11 @@ void format_trailers_from_commit(const struct process_trailer_options *opts,
 				 const char *msg,
 				 struct strbuf *out)
 {
+	LIST_HEAD(trailer_objects);
 	struct trailer_info info;
 
-	trailer_info_get(opts, msg, &info);
+	parse_trailers(opts, &info, msg, &trailer_objects);
+
 	/* If we want the whole block untouched, we can take the fast path. */
 	if (!opts->only_trailers && !opts->unfold && !opts->filter &&
 	    !opts->separator && !opts->key_only && !opts->value_only &&
@@ -1152,6 +1154,7 @@ void format_trailers_from_commit(const struct process_trailer_options *opts,
 	} else
 		format_trailer_info(opts, &info, out);
 
+	free_trailers(&trailer_objects);
 	trailer_info_release(&info);
 }