Identity comparison helpers for change records.

org.jabref.model.entry.BibEntry, org.jabref.model.entry.BibtexString and org.jabref.model.database.BibDatabase all compare by content, but a change targets one particular object: two entries with identical fields are distinct rows the user can edit independently. Records holding them therefore compare and hash by identity, so a change against one row is never considered equal to the same change against its twin.

Identity also keeps the hash stable. Content-based hashing over a mutable object -- a database's hash covers its entry list -- would change under a record that is already in the undo stack.