Got some permission to use userdiff patterns

I contacted a number of Git authors and lined up their permission
to relicense their work for use in libgit2 and copied over their
code for diff driver xfuncname patterns.  At this point, the code
I've copied is taken verbatim from core Git although Thomas Rast
warned me that the C++ patterns, at least, really need an update.
I've left off patterns where I don't feel like I have permission
at this point until I hear from more authors.