pattern-examination heuristic method to purely histogram-driven selectivity at
histogram size 100, we compute both estimates and use a weighted average.
The weight put on the heuristic estimate decreases linearly with histogram
size, dropping to zero for 100 or more histogram entries.
Likewise in ltreeparentsel().  After a patch by Greg Stark, though I
reorganized the logic a bit to give the caller of histogram_selectivity()
more control.