sparse-checkout: avoid using internal API of unpack-trees, take 2

sparse-checkout: avoid using internal API of unpack-trees, take 2

Commit 2f6b1eb794 ("cache API: add a "INDEX_STATE_INIT" macro/function,
add release_index()", 2023-01-12) mistakenly added some initialization
of a member of unpack_trees_options that was intended to be
internal-only.  This initialization should be done within
update_sparsity() instead.

Note that while o->result is mostly meant for unpack_trees() and
update_sparsity() mostly operates without o->result,
check_ok_to_remove() does consult it so we need to ensure it is properly
initialized.

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