Return a git.Tree object matching a head ref fetched into
        refs/remotes/origin/

def get_tree_from_branch(self, ref):
        '''
        Return a git.Tree object matching a head ref fetched into
        refs/remotes/origin/
        '''
        try:
            return git.RemoteReference(
                self.repo,
                'refs/remotes/origin/{0}'.format(ref)).commit.tree
        except ValueError:
            return None