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 aFileDifffrom data.
Unit tests pass on Python 2.7 and 3.7 (with other in-progress changes).