Don't panic for `uv init / --name foo` (#17983)
No test since we don't know where the test runner will have access to
`/` or not. I've manually confirmed that `uv init / --name foo` doesn't
panic anymore.
The error message for it is mediocre but passable (same as e.g. `uv init
/usr` on stable, no change just documenting it):
```
error: Failed to initialize Git repository at `/`
stdout:
stderr: /.git: Permission denied
```
