Introduce basic merging

Hey not sure if this would be useful, I needed it for something I was doing and figured it could, at the least, be useful as a base for starting to get merging in. I'm not sure about how a caller should interact with the merging stuff, so I just did a mix of how I would want to use it/how similar stuff is done. Please feel free to tear me apart because I screwed up somewhere, very new to libgit2sharp + git/github.