hook onModify into task undo (Closes: #2213)

src/TDB2.cpp

@@ -839,6 +839,9 @@ void TDB2::revert ()
     //   - erase from completed
     //   - if in backlog, erase, else cannot undo
 
+    Task old = Task (prior);
+    Context::getContext ().hooks.onModify (Task (current), old);
+
     // Modify other data files accordingly.
     std::vector <std::string> p = pending.get_lines ();
     revert_pending (p, uuid, prior);