Fix parsing of SVN diffs with property changes.

SVN diffs with property changes broke diff parsing. We now handle this
properly. We don't show the changes in the diff viewer, but we at least
don't fail miserably now.

Property changes in diffs have only recently gained any kind of
parseable format, it appears, but more work needs to be done to look
into whether we can make any of use it.

Unit tests pass.