[PATCH] Make ON CONFLICT DO NOTHING and ON CONFLICT DO UPDATE consistent

Hi hackers,

Currently we allow self-conflicting inserts for ON CONFLICT DO NOTHING:

```
CREATE TABLE t (a INT UNIQUE, b INT);
INSERT INTO t VALUES (1,1), (1,2) ON CONFLICT DO NOTHING;
-- succeeds, inserting the first row and ignoring the second
```
... but not for ON CONFLICT .. DO UPDATE:

```
INSERT INTO t VALUES (1,1), (1,2) ON CONFLICT (a) DO UPDATE SET b = 0;
ERROR: ON CONFLICT DO UPDATE command cannot affect row a second time
HINT: Ensure that no rows proposed for insertion within the same
command have duplicate constrained values.
```

Tom pointed out in 2016 that this is actually a bug [1] and I agree.

The proposed patch fixes this.

[1]: https://www.postgresql.org/message-id/22438.1477265185%40sss.pgh.pa.us

--
Best regards,
Aleksander Alekseev