Update format.rs to display correct message for already formatted files (#9153)
## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
New messages for "format" mode.
Fixes #9132
## Test Plan
<!-- How was it tested? -->
I ran the tests specified in `CONTRIBUTING.md`
```bash
cargo run -p ruff_cli -- check /path/to/some_files.py --no-cache
cargo run -p ruff_cli -- format --check /path/to/some_files.py --no-cache
cargo clippy --workspace --all-targets --all-features -- -D warnings
RUFF_UPDATE_SCHEMA=1 cargo test
pre-commit run --all-files --show-diff-on-failure
```
**Note:** In case no files are detected, either correctly formatted,
changed, or unchanged, it does not display a message. Wouldn't it be
better to show some message in this case?