Logic error in util.cpp

- taskDiff() was returning true whenever beforeOnly.size() or
  afterOnly.size() were nonzero, which made no sense and was
  clearly a typo

src/util.cpp

@@ -342,7 +342,7 @@ bool taskDiff (const Task& before, const Task& after)
   std::vector <std::string> afterOnly;
   listDiff (beforeAtts, afterAtts, beforeOnly, afterOnly);
 
-  if (beforeOnly.size () ||
+  if (beforeOnly.size () !=
       afterOnly.size ())
     return true;