Add --diff to black command

[tannewt]

That way we can hand-fix small PRs that break formatting.

[evaherrada]

@Flameeyes Do you know how we can do this now that we're using pre-commit?

[Flameeyes]

Good question — the hook documentation says not to use args, but that implies that args exists… so it would just be a matter of adding that to the hook.

But of course that would print the diff and fail on pre-commit instead of fixing it when running pre-commit locally.

[evaherrada]

@tannewt Is this still something you'd like me to try to do given that it could cause issues when running locally

[tannewt]

Can we run it twice or call regular diff afterwards?

[evaherrada]

@tannewt Yeah, I think that should work. I can try to take a look at doing this sometime soon.