Partest can --show-diff again after incremental report.

A flag is set in NestUI to look for "diff" in the transcript of failing tests.

The diff is echoed after the incremental status line for the test.

The use case is that if you ctl-c, you don't see the summary transcripts.

Sorry to annoy, I know you want to do clean-up. It's the middle of the night and I needed it.  I needed it bad.

Review or repulse by @paulp