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

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.