index cardinalities=1

Parallel repair didn't poroperly update index cardinality
in certain cases.

When myisam_sort_buffer_size is not enough to store all
keys, index cardinality was updated before index was
actually written, when no index statistic is available.

mysql-test/r/myisam.result:
  A test case for BUG#47444.
mysql-test/t/myisam.test:
  A test case for BUG#47444.
storage/myisam/sort.c:
  update_key_parts() must be called after all index
  entries are written, when index statistic is available.