format_trailers_from_commit(): indirectly call trailer_info_get()

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>