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/CmdModify.cpp

@@ -79,7 +79,7 @@ int CmdModify::execute (std::string&)
     Task before (task);
     task.modify (Task::modReplace);
 
-    if (before.data != task.data)
+    if (before.data_removeme () != task.data_removeme ())
     {
       // Abort if change introduces inconsistencies.
       checkConsistency(before, task);