gitu crash on file with ansi symbols inside

Gitu just crash on start if changes contain file with ansi symbols 

```
Error: GitDiffUtf8(FromUtf8Error { bytes: [100, 105, 102, 102, 32, 45, 45, 103, 105, 116, 32, 97,  ...
...
101, 102, 105, 110, 101, 32, 77, 79, 78, 73, 84, 79, 82, 95, 77, 79, 68, 69, 10], error: Utf8Error { valid_up_to: 13651, error_len: Some(1) } })
```