pre-push hook can check Python code

[Shunpoco]

Summary

Context: https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Run.20python.20format.20in.20pre-push.20hook/with/541794588

Currently pre-push hook kicks ./x test tidy, it means that the hook checks only Rust codes, but not Python (and other) codes.

Since we have auto prefix which skips extra check if there is no change, pre-push hook can call use it without unnecessary setup (e.g., create venv even if user doesn't touch Python code at all).

[Shunpoco]

@rustbot claim