MYSQL-364: Duplicate unique index duplicated in production

We have seen duplicate keys on unique sec. index multiple times
in past few weeks. All incidents are on t13 or above. The following
upstream bug fix seems very relevant to what we have seen.
We are going to revert the following change:

commit 79df328424d36a1dd455e92624f781ffba2dc8ae
Author: Annamalai Gurusami <annamalai.gurusami@oracle.com>
Date: Wed Mar 6 11:49:57 2013 +0530
Bug #16133801 UNEXPLAINABLE INNODB UNIQUE INDEX LOCKS ON DELETE +
INSERT WITH SAME VALUES
Problem:
When a transaction is in READ COMMITTED isolation level, gap locks are still
taken in the secondary index, when row is inserted. This happens when the
secondary index is scanned for duplicate.
The function row_ins_scan_sec_index_for_duplicate() always calls the
function row_ins_set_shared_rec_lock() with LOCK_ORDINARY irrespective of
the transaction isolation level.
Solution:
The function row_ins_scan_sec_index_for_duplicate() calls the
function row_ins_set_shared_rec_lock() with LOCK_ORDINARY or
LOCK_REC_NOT_GAP based on the transaction isolation level.
rb://2035 approved by Krunal and Marko
-BZR-
revision-id: annamalai.gurusami@oracle.com-20130306061957-kbojud8y7er0zjqs
property-audit-revid: annamalai.gurusami@oracle.com-20130306061957-4kxwqtcfsp5sshjo
property-branch-nick: mysql-5.5
testament3-sha1: 29969679d7d700616d85849bb134ba1070aeac21
Twitter MySQL 5.5.37.t16 release (with additional missing patch)