Add diffstat output to rancid diffs

[RolandRosenfeld]

Years ago Christian Hammers [email protected] suggested in https://bugs.debian.org/357218 to add a diffstat output on top of the normal diff output. This gives a quick overview if any of "my" routers are affected or just those of my collegues etc...

I extended the call by a check whether diffstat is installed (and ignore otherwise) and this little patch is available in Debian since 2007, so it doesn't seem to hurt ;-)