## Summary
Allow `respect_gitignore` even when not in a git repo
## Test Plan
Within the Ruff repository:
1. Renamed `.git` to `.hello-world`
2. Added `test.py` in root folder
3. Added `test.py` to `.gitignore`
4. Ran `cargo run --bin ruff -- check --no-cache --isolated --show-files
.` with
and without `--respect-gitignore` flag
fixes: #5930