https://github.com/openai/codex/pull/800 made `cwd` a property of `Config` and made it so the `cwd` is not necessarily `std::env::current_dir()`. As such, `is_inside_git_repo()` should check `Config.cwd` rather than `std::env::current_dir()`. This PR updates `is_inside_git_repo()` to take `Config` instead of an arbitrary `PathBuf` to force the check to operate on a `Config` where `cwd` has been resolved to what the user specified.