Being able to clone TFS path "$/"

I've become somewhat opposed to boolean args to methods. In this case, since there's only one optional argument, what if this method split in two?

```csharp
public static void AssertValidTfsPathOrRoot(this string tfsPath)
{
  if (tfsPath == GitTfsConstants.TfsRoot)
    return;
  AssertValidTfsPath(tfsPath);
}

public static void AssertValidTfsPath(this string tfsPath)
{
   // same as before
   // ...
```