The function was rewritten in GPDB, and its behaviour was changed to also
return 'true' if the expression contains an Aggref of the given level.
That change in behaviour was made back in 2006, as part of a commit
containing a lot of subquery optimization changes. I could not find an
explanation for that particular change, and all the regression tests pass
without so I assume that it has become obsolete at some point over they
years.

This smoothens the way for future merges with upstream, by reducing the
diff in both code and behaviour. Also, you get a more accurate error
message in a few cases, as seen by the changes to expected output.

Reformat some of the test queries, and make sure the formatting is the same
between the .sql file and .out file (pg_regress uses "diff -w", which masks
out some of the differences, but it was annoying while doing a manual diff).

This was done in the upstream earlier already, in commit 8b4ff8b6, but there
were a few GPDB-added error messages left. Fix those too, for consistency.

After creating view that contain ordered-set (WITHIN GROUP) aggregates, \d+ or pg_dump can not get the correct definition of the view back, the brackets around the ORDER BY clause disappear.

View definition is stored in pg_rewrite in the form of parsetree, get_rule_expr() parses the T_Percentile node back to query string format, it should append the brackets around ORDER BY clause.