SET TRANSACTION ISOLATION LEVEL is used to temporarily set
the trans.iso.level for the next transaction.  After the
transaction, the iso.level is (re-)set to value of the 
session variable 'tx_isolation'.

The bug is caused by setting the thd->variables.tx_isolation 
field to the value of the session variable upon each
statement commit.  It should only be set at the end of the
full transaction.

The fix has been to remove the setting of the variable in
ha_autocommit_or_rollback if we're in a transaction, as it 
will be correctly set in  either ha_rollback or 
ha_commit_one_phase.  

If, on the other hand, we're in  autocommit mode, tx_isolation 
will be explicitly set here.