insert...select

Queries following bulk insert into an empty MyISAM table
may break it. This was pure MyISAM problem.

When bulk insert into an empty table is complete, MyISAM
may want to enable indexes via repair by sort. If repair
by sort fails (e.g. insufficient buffer), MyISAM failover
to repair with key cache, requesting repair of data file.

Repair of data file performs data file substitution. This
means that current table instance will point to new data
file. Other cached table instances are still pointing to
an old, deleted data file.

This is fixed by not requesting repair of data file
during enable indexes.

Explicit REPAIR is not affected, since it flushes all
table instances.

mysql-test/r/myisam.result:
  A test case for BUG#51307.
mysql-test/t/myisam.test:
  A test case for BUG#51307.
storage/myisam/ha_myisam.cc:
  When enabling indexes do not attempt to repair data file.