Report number of changed files on the format CLI (#6948)
## Summary
Very basic summary:
<img width="962" alt="Screen Shot 2023-08-28 at 1 17 37 PM"
src="https://github.com/astral-sh/ruff/assets/1309177/53537aca-7579-44d8-855b-f4553affae50">
If you run with `--verbose`, we'll also show you the timing:
<img width="962" alt="Screen Shot 2023-08-28 at 1 17 58 PM"
src="https://github.com/astral-sh/ruff/assets/1309177/63cbd13e-9462-4e49-b3a3-c6663a7ad41c">
