Use portable diff options in pg_bsd_indent's regression test.

Use portable diff options in pg_bsd_indent's regression test.

We had been using "diff -upd", which evidently works for most people,
but Solaris's diff doesn't like it.  (We'd not noticed because the
Solaris buildfarm animals weren't running this test until they were
upgraded to the latest buildfarm client script.)  Change to "diff -U3"
which is what pg_regress has used for ages.

Per buildfarm (and off-list discussion with Noah Misch).

Back-patch to v16 where this test was added.  In v16,
also back-patch the relevant part of 628c1d1f2 so that
the test script looks about the same in all branches.