Inherit remote fetch replication events from a common parent

Change I29da700b introduced a common parent for all events that
represent a remote replication of a particular ref.

Inheriting from this parent allows to adhere to the provided interface,
providing a way to consume all such events in a structured way,
regardless whether they were originated by push or pull-replication.

Bug: Issue 14843
Change-Id: Ia5659d5c9445eb118d65948c5671ff19f31b57ec