R1:e5a036e8c948

Make Differential linewrap utf-8 aware

Summary: Differential uses a byte-oriented linewrap algorithm. Instead, use a character-oriented one which will handle utf-8 properly.

This implies a very slightly performance hit but we only run this code for lines which need to wrap, and the results get cached. It took about ~2.5ms for the test file on my machine. I'll keep an eye on it but I think it's currently a manageable cost.

Test Plan: Diffed this file: https://secure.phabricator.com/P43
...and got it to…