Remove -w (--ignore-all-space) option from pg_regress's diff calls.

We have used -w for a long time as a means of reducing the reported diff
volume when one element of a result table isn't of the expected width.
However, most of the time the results just pass anyway, so this isn't as
important as it once was.  Meanwhile, the risk of missing potentially
significant deviations has gone up, particularly with psql's ability to
report error cursor positions.  So, let's switch over to space-sensitive
comparisons.  Per my proposal of yesterday.

(All the expected files that I can test here seem to be ready for this
already, but we'll see what the buildfarm thinks about others.)