Update a member of a cycle with a type-preserving update, but sever the cycle.

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

``` unison
ping : 'Nat
ping _ = !pong + 1

pong : 'Nat
pong _ = !ping + 2
```

``` ucm
scratch/main> add
```

``` unison
ping : 'Nat
ping _ = 3
```

``` ucm
scratch/main> update
scratch/main> view ping pong
```