diff: teach diff to read attributes from bare repos

(a4cf900e diff: teach diff to read algorithm from diff driver, 2022-02-20) does not support bare repositories. It can be useful, especially for Git servers to utilize this feature on bare repositories.

Teach diff to check against the attributes from HEAD if a bare repository is being operated on. This change also allows custom diff drivers to work with bare repositories.

Extend this behavior by providing an option --attr-source that allows the user to specify an any commit to read gitattributes from.