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

Commit 2f6b1eb ("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>

builtin/sparse-checkout.c

@@ -217,7 +217,6 @@ static int update_working_directory(struct pattern_list *pl)
 	o.head_idx = -1;
 	o.src_index = r->index;
 	o.dst_index = r->index;
-	index_state_init(&o.result, r);
 	o.skip_sparse_checkout = 0;
 
 	setup_work_tree();

unpack-trees.c

@@ -2101,6 +2101,7 @@ enum update_sparsity_result update_sparsity(struct unpack_trees_options *o,
 
 	old_show_all_errors = o->show_all_errors;
 	o->show_all_errors = 1;
+	index_state_init(&o->result, o->src_index->repo);
 
 	/* Sanity checks */
 	if (!o->update || o->index_only || o->skip_sparse_checkout)