revlog: _addrevision creates full-replace deltas based on censored revisions

A delta against a censored revision is either received through exchange and
written blindly to a revlog, or it is created by the revlog itself. This
change ensures the latter process creates deltas which fully replace all
data in a censored base using a single patch operation.

Recipients of a delta against a censored base will verify that the delta is in
this full-replace format. Other recipients will use the delta as normal.

For background and broader design of the censorship feature, see:
http://mercurial.selenic.com/wiki/CensorPlan

mercurial/revlog.py

@@ -1261,6 +1261,12 class revlog(object):
                 delta = cachedelta[1]
             else:
                 t = buildtext()
+                if self.iscensored(rev):
+                    # deltas based on a censored revision must replace the
+                    # full content in one patch, so delta works everywhere
+                    header = mdiff.replacediffheader(self.rawsize(rev), len(t))
+                    delta = header + t
+                else:
                 ptext = self.revision(self.node(rev))
                 delta = mdiff.textdiff(ptext, t)
             data = self.compress(delta)