is a large enough histogram, it will use the number of matches in the
histogram to derive a selectivity estimate, rather than the admittedly
pretty bogus heuristics involving examining the pattern contents.  I set
'large enough' at 100, but perhaps we should change that later.  Also
apply the same technique in contrib/ltree's <@ and @> estimator.  Per
discussion with Stefan Kaltenbrunner and Matteo Beccati.