'Set a refname to new_ref only if it currently equals old_ref.
This method follows all symbolic references if applicable for the
subclass, and can be used to perform an atomic compare-and-swap
operation.
:param name: The refname to set.
:param old_ref: The old sha the refname must refer to, or None to set
unconditionally.
:param new_ref: The new sha the refname will refer to.
:return: True if the set was successful, False otherwise.'

def set_if_equals(self, name, old_ref, new_ref):

    raise NotImplementedError(self.set_if_equals)
