yapf-diff: Incremental python formatting

A tool to help my team incrementally adopt consistent python style by formatting only changed lines.

It might be a character flaw, but I can’t stand inconsistent style. To avoid wasting time in code review talking about style, I initially proposed using black. My team was hesitant to lose all the blame on our thousands of lines of python, so I wrote a YAPF style guide and this command-line tool to format only what was changed in a PR.