parendelta: fix computation of base rev (fixes issue2337) Refactor revlog._addrevision() and put the correct base rev in the parent-delta case: base(rev) should always be equal to the first full snapshot that is needed by the delta chain, in both parent-delta and tip-delta case. Before this fix, the base rev was in most case wrong (and in the case where p1 == nullid, this triggered the bug from issue2337). This means that repositories converted to parent-delta earlier are corrupted and needs to be reconverted.