Using --parallel together with --diff produces incorrect output

Running yapf 0.24.0 on a larger codebase with `--parallel` and `--diff` produces incorrect output, it looks like there is some missing lock: The diffs from different files are sometimes mixed together.

By the very nature of the problem, it's a bit tricky to give detailed reproduction steps. Nevertheless, it basically always happens on our internal codebase with roughly 2100 Python files, where lots of them actually result in a diff. You can e.g. grep the output for the diff headers (`+++ ` and `--- `), and see a handful of them broken.

I'm quite aware of the fact that this is not a perfect issue description, but I don't know how to give a better repro. If there is something I can do/test/modify locally, please let me know.