revlog: implement sidedata without using _revisiondata

When they was introduced sidedata where grouped with the actual revision data
and unpacking one came with the other. Sidedata moved be stored "independently"
and it no longer make sense to retrieve both at the same time unconditionnaly.
We start with changeset the implementation of the `revlog.sidedata` command to
no longer use `self._revisiondata`. More users need to be migrated to direct
usage of this `revlog.sidedata` method. This will be done in the coming
changesets.

Differential Revision: https://phab.mercurial-scm.org/D10780

mercurial/revlog.py

@@ -1976,7 +1976,12 b' class revlog(object):'
         mapping object will likely be used in the future for a more
         efficient/lazy code.
         """
-        return self._revisiondata(nodeorrev, _df)[1]
+        # deal with <nodeorrev> argument type
+        if isinstance(nodeorrev, int):
+            rev = nodeorrev
+        else:
+            rev = self.rev(nodeorrev)
+        return self._sidedata(rev)
 
     def _revisiondata(self, nodeorrev, _df=None, raw=False):
         # deal with <nodeorrev> argument type