Using ruff check diff suppresses errors

Hey, thanks for the great work.
I don't know if this is a bug or maybe unclear flag, but doing `ruff check --diff .` does not fail:

```py
try:
    pass
except:
    pass
```

But running `ruff check .` shows proper errors:

```
E722 Do not use bare `except`
```

Is this expected? I didn't expect `--diff` to have this side-effect, maybe the help is not clear?