gui: Fix undefined behavior in TreeModel.

std::sort() requires the comparison function to return false for even
comparison. Returning true results in undefined behavior and a potential
segfault.

gui/treemodel.cc

@@ -135,7 +135,7 @@ void IdStringList::updateElements(Context *ctx, std::vector<IdString> elements)
         }
 
         // Same string.
-        return true;
+        return false;
     });
 }