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).
FWIW there are many tests that depend on the current `repo.Head.Tip`, including some `Reset()` tests. No harm in depending on it here too, IMO.