Failing to do this results in almost all updates to system catalogs
being non-HOT updates, because the OID column would differ (not having
been set for the new tuple), which is an indexed column.

While at it, make sure to set the tableoid early in both old and new
tuples as well.  This isn't of much consequence, since that column is
seldom (never?) indexed.

Report and patch from Andres Freund.