Leading whitespace in a file isn't removed after formatting if code is indented even 1 character

Code:
```rust

    use core::cmp::max;
```
(empty line, then 4 spaces and a `use`, github does not display this correctly for me)

after first run of `rustfmt`:
```
$ echo -e "\n    use core::cmp::max;" | rustfmt

use core::cmp::max;
```
(notice the blank line before the `use`)

after the second run:
```
$ echo -e "\n    use core::cmp::max;" | rustfmt | rustfmt
use core::cmp::max;
```