trailer: use list.h for doubly-linked list

Replace the existing handwritten implementation of a doubly-linked list
in trailer.c with the functions and macros from list.h. This
significantly simplifies the code.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jonathantanmy