merge-ort: pass repository to write_tree()

merge-ort: pass repository to write_tree()

In order to get rid of a usage of the_repository, we need to know the
value of opt->repo; pass it along to write_tree().  Once we have the
repository, though, we no longer need to pass
opt->repo->hash_algo->rawsz, we can have write_tree() look up that value
itself.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>