Use best holdout loss for reporting final 'average loss' when holdout enabled

Even with multiple passes the 'average loss' in final output was still
progressive validation which does not make sense.

- This pull-request shows best holdout loss if holdout is enabled with a
  customary 'h' appended.
- hit the too small diff width issue in testcases again and while debugging I
  have switched the printed diff to the same side-by-side diff format that is
  used internally for lenient float comaprison. This made differences more
  obvious and debugging easier.
- disabled holdout in the affix set which did not make sense to me as there
  rare just 6 examples.