hidden: Add a hiddenrevs attributes to changelog.

This attributes hold the set of all revisions that should be ommited by command
and tools displaying changesets.

This set is given as a hit. Command and tools are responsible to check it in
order to filter they outpur.

Code adding revisions to the set are responsible to the consistency of it's
data.

mercurial/changelog.py

@@ -107,6 +107,8 b' class changelog(revlog.revlog):'
         self._realopener = opener
         self._delayed = False
         self._divert = False
+        # hiddenrevs: revs that should be hidden by command and tools
+        self.hiddenrevs = set()
 
     def delayupdate(self):
         "delay visibility of index updates to other readers"