revlog: add "iscensored()" to revlog public API

The iscensored method will be used by the exchange layer to reject
nonconforming deltas involving censored revisions (and to produce
conforming deltas).

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

mercurial/filelog.py

@@ -64,7 +64,7 b' class filelog(revlog.revlog):'
         node = self.node(rev)
         if self.renamed(node):
             return len(self.read(node))
-        if self._iscensored(rev):
+        if self.iscensored(rev):
             return 0
 
         # XXX if self.read(node).startswith("\1\n"), this returns (size+4)
@@ -85,7 +85,7 b' class filelog(revlog.revlog):'
             return False
 
         # censored files compare against the empty file
-        if self._iscensored(self.rev(node)):
+        if self.iscensored(self.rev(node)):
             return text != ''
 
         # renaming a file produces a different hash, even if the data
@@ -104,6 +104,6 b' class filelog(revlog.revlog):'
                 raise error.CensoredNodeError(self.indexfile, node)
             raise
 
-    def _iscensored(self, rev):
+    def iscensored(self, rev):
         """Check if a file revision is censored."""
         return self.flags(rev) & revlog.REVIDX_ISCENSORED

mercurial/revlog.py

@@ -1419,6 +1419,10 b' class revlog(object):'
 
         return content
 
+    def iscensored(self, rev):
+        """Check if a file revision is censored."""
+        return False
+
     def getstrippoint(self, minlink):
         """find the minimum rev that must be stripped to strip the linkrev
 

mercurial/unionrepo.py

@@ -160,6 +160,12 b' class unionfilelog(unionrevlog, filelog.'
     def baserevdiff(self, rev1, rev2):
         return filelog.filelog.revdiff(self, rev1, rev2)
 
+    def iscensored(self, rev):
+        """Check if a revision is censored."""
+        if rev <= self.repotiprev:
+            return filelog.filelog.iscensored(self, rev)
+        return self.revlog2.iscensored(rev)
+
 class unionpeer(localrepo.localpeer):
     def canpush(self):
         return False