revlog: special case expanding full-replacement deltas received by exchange When a delta received through exchange is added to a revlog, it will very often be expanded to a full text by applying the delta to its base. If that delta is of a particular form, we can avoid decoding the base revision. This avoids an exception if the base revision is censored. For background and broader design of the censorship feature, see: http://mercurial.selenic.com/wiki/CensorPlan