Yeah, it turns out that if `compareOptions == null` should behave the same as `compareOptions == new CompareOptions()`, you have to specify `None` here.

Regarding test coverage: what's our goal here? Since Libgit2sharp's value is in making libgit2 available to the CLR in an idiomatic way, it seems like the tests should just be verifying that changing the option changes the behavior in the way it's documented. I'm not sure there's much value in trying to verify all the corner cases of the libgit2 diff implementation, especially since libgit2 already has a test suite for that.