Require diffs set on a FileDiff to be byte strings.
Review Request #10499 — Created April 3, 2019 and submitted — Latest diff uploaded
FileDiffwas previously allowing either byte strings or Unicode
strings, which created inconsistency. We now require byte strings, and
check for this when creating a
Unit tests pass on Python 2.7 and 3.7 (with other in-progress changes).