self-join

When doing DELETE with self-join on a MyISAM or MERGE table, it could
happen that a record being retrieved in join_read_next_same() has
already been deleted by previous iterations. That caused the engine's
index_next_same() method to fail with HA_ERR_RECORD_DELETED error and
the whole DELETE query to be aborted with an error.

Fixed by suppressing the HA_ERR_RECORD_DELETED error in
hy_myisam::index_next_same() and ha_myisammrg::index_next_same(). Since
HA_ERR_RECORD_DELETED can only be returned by MyISAM, there is no point
in filtering this error in the SQL layer.


mysql-test/r/merge.result:
  Added a test case for bug #28837.
mysql-test/r/myisam.result:
  Added a test case for bug #28837.
mysql-test/t/merge.test:
  Added a test case for bug #28837.
mysql-test/t/myisam.test:
  Added a test case for bug #28837.
sql/ha_myisam.cc:
  Skip HA_ERR_RECORD_DELETED silently when calling mi_rnext_same().
sql/ha_myisammrg.cc:
  Skip HA_ERR_RECORD_DELETED silently when calling mi_rnext_same().