When dealing with multiple grouping columns coming from the same table,

clamp the estimated number of groups to table row count over 10, instead
of table row count; this reflects a heuristic that people probably won't
group over a near-unique set of columns, and the knowledge that we don't
currently have any way to estimate the correlation of the columns better
than guessing.  This change creates a trivial plan change in one of the
regression tests.