Binary operator can duplicate itself in certain situations in a macro

Example code:

```rust
m!(a: b!(c!) % f);
```

Is formatted into:

```rust
m!(a: b!(c!) % % f);
```

Which is not identical.

Noticed with https://github.com/xfix/advent-of-code-2017/blob/master/day18/src/main.rs with `>>` operator, and simplified.

This happens on playpen.