R1:5cfc14cb43c5

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…