Make a small improvement to unicode diffs in 1.7.x

Generally, Review Board 2.0's support for unicode is much, much better than
1.7.x, but this is a small enough change that it's probably worth making. Some
of the filename processing can do operations which will fail if the filenames
an un-decoded utf8, so convert all filenames from diffs to unicode if the
SCMTool didn't already.

Testing done: Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/5784/