rawdata: introduce a `rawdata` method on revlog

This method aims at replacing `revision(..., raw=True)` call. The purpose of
data returned without and without raw are different enough that having two
different method would make sense.

This split is motivated by other work aiming at storing data on the side of the
main revision of a revlog. Having a cleaner API makes it simpler to add this
work.

The series following this first changesets is organised as follow:
1) add `rawdata` method everywhere it is useful
2) update all caller
3) implement all `rawdata` method without using `revision`
4) deprecate the `rawdata` parameter of `revision`

mercurial/revlog.py

@@ -1720,6 +1720,13 b' class revlog(object):'
 
         return text
 
+    def rawdata(self, nodeorrev, _df=None, raw=False):
+        """return an uncompressed raw data of a given node or revision number.
+
+        _df - an existing file handle to read from. (internal-only)
+        """
+        return self._revisiondata(nodeorrev, _df, raw=True)
+
     def hash(self, text, p1, p2):
         """Compute a node hash.