Recursive output poorly reported

We have a case where we have a linked datastructure pointing to both children and parent nodes. Due to a bug in our test implementation we were getting a result that seems like a false-positive, but in fact `cmp` hides the actual difference under `ref#0`. This occurrs because the data structure has the same values, but different pointer addresses. See the simplified example input and output below:

```go
func Test_demo(t *testing.T) {
type node struct {
parent *node
child *node
}

t2 := &node{}
t1 := &node{child: t2}
t2.parent = t1

t3 := &node{child: t2}
t2.parent = t3

if diff := cmp.Diff(t1, t3, cmp.AllowUnexported(node{})); diff != "" {
t.Errorf("node mismatch (-want +got):\n%s", diff)
}
}
```

which produces the following output:
```
demo_test.go:20: node mismatch (-want +got):
&⟪ref#0⟫demo.node{
parent: nil,
child: &demo.node{
- parent: &⟪ref#0: 0x0140004009b0⟫(...),
+ parent: &⟪ref#0: 0x0140004009b0⟫(...),
child: nil,
},
}
```

The example code is obviously flawed, but in our case that was hidden among 100 other lines before it was distilled to these lines