fix: merge tuple transformation didn't group cols

- merge tuple transformation that merges several
and-ed equalities into equlities of rows didn't
group columns by child they refer to. This led
to rows where we couldn't find sharding keys,
because they were scattered across the different
rows:

```
sk(t1) = (a, b), sk(t2) = (e, f)
... on (t1.a, t2.f) = (t2.e, t1.b)
```

But now correct rows are generated:

```
... on (t1.a, t1.b) = (t2.e, t2.f)
```