context: rewrite renamed() in terms of new copysource() where appropriate

It's only the filectx for committed files that have the copy source's
nodeid easily available; workingfilectx's and overlayfilectx's
renamed() simply find the nodeid by looking in the changeset's p1's
manifest.

Differential Revision: https://phab.mercurial-scm.org/D6009

mercurial/context.py

@@ -1701,7 +1701,7 b' class workingfilectx(committablefilectx)'
     def data(self):
         return self._repo.wread(self._path)
     def renamed(self):
-        rp = self._repo.dirstate.copied(self._path)
+        rp = self.copysource()
         if not rp:
             return None
         return rp, self._changectx._parents[0]._manifest.get(rp, nullid)
@@ -2147,7 +2147,7 b' class overlayworkingfilectx(committablef'
         return self._parent.exists(self._path)
 
     def renamed(self):
-        path = self._parent.copydata(self._path)
+        path = self.copysource()
         if not path:
             return None
         return path, self._changectx._parents[0]._manifest.get(path, nullid)