Nested block inside closure has no fixpoint

In 0.8.4, 
```rust
fn foo() {
    let f = |x| {{x}};
}
```
will be formatted to
```rust
fn foo() {
    let f = |x| {
        x
    };
}
```
and then
```rust
fn foo() {
    let f = |x| x;
}
```