overlayworkingctx: fix a bad reference to `self._path`

Differential Revision: https://phab.mercurial-scm.org/D1236

mercurial/context.py

@@ -2010,7 +2010,7 b' class overlayworkingctx(workingctx):'
                     return self._wrappedctx[path].data()
             else:
                 raise error.ProgrammingError("No such file or directory: %s" %
-                                             self._path)
+                                             path)
         else:
             return self._wrappedctx[path].data()