Change pg_regress.c back to using diff -w on Windows

Change pg_regress.c back to using diff -w on Windows

This partially reverts commit 628c1d1f2c.

It appears that there are non line-end differences in some regression
tests on Windows. To keep the buildfarm and CI clients happy, change
this back for now, pending further investigation.

Per reports from Tatsuo Ishii and Nazir Bilal Yavuz.