addrevision: only use the incoming base if it is a good delta (issue4975) Before this change, the 'lazydeltabase' would blindly build a delta using the base provided by the incoming bundle and try to use it. If that base was far down the revlog, the delta would be seen as "no good" and we would fall back to a full text revision. We now check if the delta is good and fallback to a computing a delta again the tipmost revision otherwise (as we would do without general delta). Later changesets will improve the logic to compute the fallback delta using the general delta logic.