trailer: make args have their own struct

trailer: make args have their own struct

Improve type safety by making arguments (whether from configuration or
from the command line) have their own "struct arg_item" type, separate
from the "struct trailer_item" type used to represent the trailers in
the buffer being manipulated.

This change also prepares "struct trailer_item" to be further
differentiated from "struct arg_item" in future patches.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>