pgsql: Fix ndistinct estimates with system attributes

Fix ndistinct estimates with system attributes

When estimating the number of groups using extended statistics, the code
was discarding information about system attributes. This led to strange
situation that

SELECT 1 FROM t GROUP BY ctid;

could have produced higher estimate (equal to pg_class.reltuples) than

SELECT 1 FROM t GROUP BY a, b, ctid;

with extended statistics on (a,b). Fixed by retaining information about
the system attribute.

Backpatch all the way to 10, where extended statistics were introduced.

Author: Tomas Vondra
Backpatch-through: 10

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/88acb0135720076599308e6fd58b659c81d1beba

Modified Files
--------------
src/backend/utils/adt/selfuncs.c | 6 +++---
src/test/regress/expected/stats_ext.out | 2 +-
2 files changed, 4 insertions(+), 4 deletions(-)