docs: 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