Explicitly say "diff cannot be shown for binary files" when showing diff for binary files.

As the title suggests, in the diff screen do not just show nothing for binary files. Say explicitly that the diff cannot be shown because the file is binary.

It's not always obvious what files are treated as binary, which can create false impression that there's no diff. This gets even worse with text files using UTF-16 and UTF-32 encoding, that are currently seen as binary by Bitbucket.