pgsql: Repair misbehavior with duplicate entries in FK SET column lists

Repair misbehavior with duplicate entries in FK SET column lists.

Since v15 we've had an option to apply a foreign key constraint's
ON DELETE SET DEFAULT or SET NULL action to just some of the
referencing columns. There was not a check for duplicate entries in
the list of columns-to-set, though. That caused a potential memory
stomp in CreateConstraintEntry(), which incautiously assumed that
the list of columns-to-set couldn't be longer than the number of key
columns. Even after fixing that, the case doesn't work because you
get an error like "multiple assignments to same column" from the SQL
command that is generated to do the update.

We could either raise an error for duplicate columns or silently
suppress the dups, and after a bit of thought I chose to do the
latter. This is motivated by the fact that duplicates in the FK
column list are legal, so it's not real clear why duplicates
in the columns-to-set list shouldn't be. Of course there's no
need to actually set the column more than once.

I left in the fix in CreateConstraintEntry() too, just because
it didn't seem like such low-level code ought to be making
assumptions about what it's handed.

Bug: #18879
Reported-by: Yu Liang <luy70(at)psu(dot)edu>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/18879-259fc59d072bd4d7@postgresql.org
Backpatch-through: 15

Branch
------
REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f5069f0264eedd71ee138b872b131a2974017b6a

Modified Files
--------------
src/backend/catalog/pg_constraint.c | 3 ++-
src/backend/commands/tablecmds.c | 35 ++++++++++++++++++++++++-------
src/test/regress/expected/foreign_key.out | 3 ++-
src/test/regress/sql/foreign_key.sql | 3 ++-
4 files changed, 34 insertions(+), 10 deletions(-)