revlog: split a `_revisiondata` method to file `revision` job

We are about to introduce more public method to access revision data (eg:
`rawdata`). revset subclass tend to recursively call `revision` which will
create all kind of issue with the coming series. To avoid them we introduce an
explicit difference between the internal call and the public all. This will be
useful for later work anyway (so the subclass issue is just moving it earlier in
the series). I am not sure if the subclass are actually doing something
sensible. However, I am certain I don't want to be rabbit holed into figuring it
out right now.

mercurial/revlog.py

@@ -1651,6 +1651,9 b' class revlog(object):'
         treated as raw data when applying flag transforms. 'raw' should be set
         to True when generating changegroups or in debug commands.
         """
+        return self._revisiondata(nodeorrev, _df, raw=raw)
+
+    def _revisiondata(self, nodeorrev, _df=None, raw=False):
         if isinstance(nodeorrev, int):
             rev = nodeorrev
             node = self.node(rev)