Add built in diff drivers

This extends the diff driver parser to support multiline driver definitions along with `!` prefixing for negated matches.  The old implementation did not match core Git in this regard, which was not ideal for config-based drivers, but which completely precluded supporting the built in diff drivers.

This also adds an internal table of driver definitions and a fallback code path that will look in that table for diff drivers that are set with attributes without having a definition in the config file.

I have yet to decide which drivers I will bring over from core Git.

- [x] multi-line function name definitions
- [x] negative matches for function name definitions
- [x] fallback to internal table of diff drivers for missing drivers
- [x] added match extraction from pattern diff matches
- [x] decide which diff drivers we want to take from Git (maybe just all of them)
- [x] copy over diff drivers from Git
- [x] talk about relicensing of diff drivers from Git
- [x] test some of the ported drivers