Tests a variable capture problem.

After pattern compilation, the match would end up:

    T p1 p3 p3

and z would end up referring to the first p3 rather than the second.

```ucm:hide
scratch/main> builtins.merge
```

```unison
structural type Trip = T Nat Nat Nat

bad : Nat -> (Nat, Nat)
bad x = match Some (Some x) with
  Some (Some x) -> match T 3 4 5 with
    T _ _ z -> (x, z)
  _ -> (0,0)

> bad 2
```