myisam/mi_rnext_same.c:
  Fix for BUG#6699: 
    MERGE handler now uses mi_rnext_same() with priority queue, so skip record unpacking if buf==NULL
myisammrg/myrg_rnext_same.c:
  Fix for BUG#6699: 
   make myrg_rnext_same sort always sort records and return them in key order.
mysql-test/r/merge.result:
  Test for BUG#6699
mysql-test/t/merge.test:
  Test for BUG#6699