Ease the creation of unborn branches

[nulltoken]

Currently, the way to do this is the following

repo.Refs.UpdateTarget("HEAD", "refs/heads/unborn");

However, the following overload cringes as the passed in reference canonical name doesn't point to an existing GitObject. Let's loosen this behavior so that it deals with Head in a specific way.

repo.Refs.UpdateTarget(repo.Refs.Head, "refs/heads/unborn");