addrevision: rework generaldelta computation The old code have multiple explicit tests and code duplications. This makes it hard to improve the code. We rewrite the logic in a more generic way, not changing anything of the computed result. The final goal here is to eventually be able to: - factor out the default fallback case "try against 'prev'" in a single place - allow 'lazydeltabase' case to use the smarter general delta code path when the incoming base does not provide us with a good delta.