Natively show ASCII-8BIT diffs if git supports them

As mentioned in #207124 (closed), diffs with ASCII-8BIT encoding need to be converted to UTF-8 in order to work with our current diff setup (showing them as JSON, specifically)

This could result in some loss of fidelity if there are unconvertable characters. GitLab should ideally be able to show these diffs regardless of encoding as long as git can show the diff.