pgsql: When dealing with multiple grouping columns coming from the same

Log Message:
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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.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        selfuncs.c (r1.169 -> r1.170)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.169&r2=1.170)
    pgsql/src/test/regress/expected:
        subselect.out (r1.10 -> r1.11)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/subselect.out.diff?r1=1.10&r2=1.11)