Comment 1 for bug 764108

Still to do on this:

 * Ideally, rewrite it so that the function prefixes work with internal diff. Otherwise if this is turned on in a rules file, it's likely to break anyone who checks out that tree on Windows or some other platform without gnu diff, and there might be other side effects.

 * Document the new rule setting in the rules help

 * Also document it in the user guide

 * Spiv mentions the issue of its effect on diffs used in bundle generation, shelves, etc. I think it'll be harmless there, and perhaps even useful to see them inline. It shouldn't break anything, but we should probably at least test it.