xdiff users: use designated initializers for out_line

xdiff users: use designated initializers for out_line

Amend the code added in 611e42a5980 (xdiff: provide a separate emit
callback for hunks, 2018-11-02) to be more readable by using
designated initializers.

This changes "priv" in rerere.c to be initialized to NULL as we did in
merge-tree.c. That's not needed as we'll only use it if the callback
is defined, but being consistent here is better and less verbose.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>