context: reimplement memfilectx.cmp()

If I added a sanity check to basefilectx, test-context.py exploded. This
patch copies the naive implementation from overlayworkingfilectx.

mercurial/context.py

@@ -2292,6 +2292,9 b' class memfilectx(committablefilectx):'
         if copied:
             self._copied = (copied, nullid)
 
+    def cmp(self, fctx):
+        return self.data() != fctx.data()
+
     def data(self):
         return self._data