Ease the creation of unborn branches

Currently, the way to do this is the following

``` csharp
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.

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