! Parallel -V output has some mixing [bug karen]

  Need to audit that all diagnostics is issued atomically via diag_record.

- Save diff output for {stdout,stderr}.diff [feature karen]

  Now we have diff output first, then the error. This often messes up emacs
  error navigation. Also, not atomic.

  So instead we should save the output to a .diff file (could be useful for
  later examination), print the error (including an info line with where is
  the diff), and then (a fragment of) the diff. Theoretically, the .diff
  can be large, so probably makes sense to limit it to a few KB?