Add version to repo object store, set repo head to version sha.

        :param message: Message string.

def save(self, message):
        """
        Add version to repo object store, set repo head to version sha.

        :param message: Message string.
        """
        self.commit.message = message
        self.commit.tree = self.tree
        #TODO: store new blobs only
        for item in self.tree.items():
            self.repo.object_store.add_object(item.blob)
        self.repo.object_store.add_object(self.tree)

        # set HEAD to new commit
        self.repo.object_store.add_object(self.commit)
        self.repo.refs['refs/heads/master'] = self.commit.id