revlog: use `self.sidedata` directly in `revlog.clone`

We want to change `revlog._revisiondata` signature, so we need to migrate users
away.

Differential Revision: https://phab.mercurial-scm.org/D10782

mercurial/revlog.py

@@ -3109,7 +3109,8 b' class revlog(object):'
             cachedelta = None
             rawtext = None
             if deltareuse == self.DELTAREUSEFULLADD:
-                text, sidedata = self._revisiondata(rev)
+                text = self._revisiondata(rev)[0]
+                sidedata = self.sidedata(rev)
 
                 if sidedata_helpers is not None:
                     (sidedata, new_flags) = sidedatautil.run_sidedata_helpers(
@@ -3137,7 +3138,8 b' class revlog(object):'
 
                 sidedata = None
                 if not cachedelta:
-                    rawtext, sidedata = self._revisiondata(rev)
+                    rawtext = self._revisiondata(rev)[0]
+                    sidedata = self.sidedata(rev)
                 if sidedata is None:
                     sidedata = self.sidedata(rev)