ruff

b8d378b0 - Add a script that tests formatter stability on repositories (#5055)

Add a script that tests formatter stability on repositories (#5055) ## Summary We want to ensure that once formatted content stays the same when formatted again, which is known as formatter stability or formatter idempotency, and that the formatter prints syntactically valid code. As our test cases cover only a limited amount of code, this allows checking entire repositories. This adds a new subcommand to `ruff_dev` which can be invoked as `cargo run --bin ruff_dev -- check-formatter-stability <repo>`. While initially only intended to check stability, it has also found cases where the formatter printed invalid syntax or panicked. ## Test Plan Running this on cpython is already identifying bugs (https://github.com/astral-sh/ruff/pull/5089)