Diagnostic doesn't align properly in span where line number increases by one digit

For example, in this case there should be two spaces between `8` and `fn` instead of one.

``` rust
test2.rs:8:1: 12:2 warning: code is never used: `foo`, #[warn(dead_code)] on by default
test2.rs:8 fn foo() {
test2.rs:9 
test2.rs:10 
test2.rs:11 
test2.rs:12 }
```
