Reset / Unstage renamed items

Currently, if you try Repository.Reset() when you have renamed entries, it chokes as `Index.Reset` expects only certain types of `ChangeKind`s.  It's easiest to treat renamed entries exactly like git core does, which is to say to treat them as an add and a delete in `git-reset` and allow you to reset each side independently (which then breaks the facade of a rename).
If that were to happen, I imagine this test would need to be updated anyways, as you would be reseting the index to a commit other than HEAD, and it would probably have other staged changes at that point?

Anyway, not a big deal. I do think a comment describing what the intent of why you are using that specific commit might help when looking at this code in the future.