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 <[email protected]>
Signed-off-by: Ramsay Jones <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jonathantanmy