Fix cost_rescan() to account for multi-batch hashing correctly.

cost_rescan assumed that we don't need to rebuild the hash table when
rescanning a hash join.  However, that's currently only true for
single-batch joins; for a multi-batch join we must charge full freight.

This probably has escaped notice because we'd be unlikely to put a hash
join on the inside of a nestloop anyway.  Nonetheless, it's wrong.
Fix in HEAD, but don't backpatch for fear of destabilizing plans in
stable releases.