context: error out if basefilectx.cmp() is called without self._filenode

The base implementation can't handle such cases because the filelog has no
knowledge about the working directory.

Loading self._filenode should have no extra cost since self.size() would
load it anyway.

mercurial/context.py

@@ -702,6 +702,10 b' class basefilectx(object):'
         if fctx._customcmp:
             return fctx.cmp(self)
 
+        if self._filenode is None:
+            raise error.ProgrammingError(
+                'filectx.cmp() must be reimplemented if not backed by revlog')
+
         if fctx._filenode is None:
             if self._repo._encodefilterpats:
                 # can't rely on size() because wdir content may be decoded