Replace direct Task.data access with a temp getter (syntactic refactor)

Now this getter, `data_removeme`, can easily be grepped out and
replaced, one usage at a time in small PRs.

src/commands/CmdDenotate.cpp

@@ -128,7 +128,7 @@ int CmdDenotate::execute (std::string&)
       }
     }
 
-    if (before.data != task.data)
+    if (before.data_removeme () != task.data_removeme ())
     {
       auto question = format ("Denotate task {1} '{2}'?",
                               task.identifier (true),