Every subsequent query to a merge table with indexes was lowering
down cardinality.

The problem was that key statistics was not cleared when merge
children were detached. Causing next attach children perform
incremental key statistics calculation.

Fixed by clearing key statistics when attaching first child.

mysql-test/r/merge.result:
  A test case for BUG#39185.
mysql-test/t/merge.test:
  A test case for BUG#39185.
storage/myisammrg/myrg_open.c:
  Clear key statistics when we're attaching first child, even
  if it's buffer was allocated before. This is needed because
  detach_children() doesn't clear statistics, causing incremental
  statistics calculation.