EXPLAIN actual ascending nibble query to get MySQL's chosen index, and use that instead of the tool's chosen index. TODO: fix OobNibbleIterator.t and chunk_index.t.

t/pt-table-checksum/samples/not-using-pk-bug.sql

@@ -0,0 +1,20 @@
+DROP DATABASE IF EXISTS test;
+CREATE DATABASE test;
+USE test;
+CREATE TABLE `multi_resource_apt` (
+  `apt_id` int(10) unsigned NOT NULL DEFAULT '0',
+  `res_id` int(10) unsigned NOT NULL DEFAULT '0',
+  PRIMARY KEY (`apt_id`,`res_id`),
+  KEY `resid` (`res_id`)
+) ENGINE=InnoDB;
+INSERT INTO multi_resource_apt VALUES
+  (1, 1),
+  (2, 1),
+  (2, 2),
+  (3, 1),
+  (3, 2),
+  (3, 3),
+  (4, 1),
+  (4, 2),
+  (4, 3),
+  (4, 4);