Make a small improvement to unicode diffs in 1.7.x
Review Request #5784 — Created May 8, 2014 and submitted
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.
Ran unit tests.
