combine-diff: convert diff_tree_combined to struct object_id

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/diff.c b/builtin/diff.c
index b2d7c32..73b4ff3 100644
--- a/builtin/diff.c
+++ b/builtin/diff.c
@@ -194,7 +194,7 @@ static int builtin_diff_combined(struct rev_info *revs,
 		revs->dense_combined_merges = revs->combine_merges = 1;
 	for (i = 1; i < ents; i++)
 		oid_array_append(&parents, &ent[i].item->oid);
-	diff_tree_combined(ent[0].item->oid.hash, &parents,
+	diff_tree_combined(&ent[0].item->oid, &parents,
 			   revs->dense_combined_merges, revs);
 	oid_array_clear(&parents);
 	return 0;