Fix post-commit diff creation with unicode characters.
The post-commit diff fetch was returning a unicode object, but the guts of all

our diff stuff (Base64Field, hashlib.sha1) expected a str of bytes. This meant

that trying to use post-commit for any change that involved non-ascii

characters would fail with an exception on the backend (and an infinite spinner

on the front). We now explicitly encode the diff to utf-8 before creating the

review request, which fixes these issues.

Created some post-commit review requests for changes that previously failed.