for ENGINE=MRG_MYISAM (should be optimized out).

Before WL#3281 MERGE engine had the HA_NOT_EXACT_COUNT flag
unset, and it worked with COUNT optimization as desired.
After the removal of the HA_NOT_EXACT_COUNT flag neither
HA_STATS_RECORDS_IS_EXACT (opposite to former HA_NOT_EXACT_COUNT
flag) nor modern HA_HAS_RECORDS flag were not added to MERGE
table flag mask.

1. The HA_HAS_RECORDS table flag has been set.
2. The ha_myisammrg::records method has been overridden to
   calculate total number of records in underlying tables.