Implement the logic to process trailers from the input message
and from arguments.
At the beginning trailers from the input message are in their
own "in_tok" doubly linked list, and trailers from arguments
are in their own "arg_tok" doubly linked list.
The lists are traversed and when an "arg_tok" should be "applied",
it is removed from its list and inserted into the "in_tok" list.
Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>