Attached is v4 of the patch series.  Changes from v3:

   * Don't process stderr if not prefixing stderr (stderr and stdout 
will refer to the same underlying file if --merge is passed, preserving 
the previous behavior)
   * Flush each line of stdout when prefixing to hopefully reduce 
out-of-order processing of stderr vs. stdout.
   * Document that --stderr-prefix can cause out-of-order processing, 
even with --merge.
   * Re-wrap comments at column 80.
   * Use `test ...` instead of `[ ... ]`.
   * Add tests for added terminating newline on final stderr line.
   * Add some additional comments to the new tests to make them easier 
to read.