Introduce RemoteCollection.Rename.

Ready to merge on sight!
How about **[this](https://github.com/libgit2/libgit2sharp/blob/eb08934e9f914975f45eb8ce9c8d29c8dea69dfa/LibGit2Sharp.Tests/RefSpecFixture.cs#L82-L83)** in `RefSpecFixture.cs`?