appending a pattern to the default "diff.cpp.xfuncname"

Hi,

I've used diff.<TYPE>.xfuncname with great success for file <TYPE>s that
I defined myself. However, now I would like to append an extra pattern
to the TYPE=cpp case (for which git has builtin patterns). Is there an
easy way to do this?

I figured I could open-code the builtin patterns from "userdiff.c", and
then append my new pattern to those, but it looks kinda gross :)

Thanks!
Laszlo