support refs with or without 'refs/' prefix

The git ref methods were mostly written to not expect the 'refs/' prefix
on ref names, even though it is included the ref.Ref value returned by
GitHub.  This resulted in ref values returned from some methods that
couldn't be passed to other methods.  In reality, ref names passed to
these methods should normally include the prefix, but this change
supports either form for backwards compatibility.

Fixes #133