Recanonicalise regression.sh whitespace -- USE -b/-w TO DIFF/BLAME

This file had mostly 8-space tabs, which previously went unnoticed.
See notes in fcb8af08a6fd3bfcf195407c4ba5bae528ee326d.
Set output file threading only if there is an output file

In particular, there is no output file with -c.  Fixes #302.