Slightly improve UTF-8 handling in Differential

Summary:
See comments. I think this will fix the issue, where we end up handling off
garbage to htmlspecialchars() after highlighting a file we've stuck full of \0
bytes.

The right fix for this is to make wordwrap and intraline-diff utf8 aware and
throw this whole thing away. I'll work on that but I think this fixes the
immediate issue.

Test Plan:
diffed the file with a UTF-8 quote in it and got a reasonable render in
Differential

Reviewed By: jungejason
Reviewers: jungejason, aran, tuomaspelkonen
CC: aran, jungejason
Differential Revision: 504