In UpdateStats(), don't bother to update the pg_class row if it already

In UpdateStats(), don't bother to update the pg_class row if it already
contains the correct statistics.  This is a partial solution for the
problem of allowing concurrent CREATE INDEX commands: unless they commit
at nearly the same instant, the second one will see the first one's
pg_class updates as committed, and won't try to update again, thus
avoiding the 'tuple concurrently updated' failure.