`set_properties_after_merging` fails to set `is_merged` when merging name strategy is `keep`

`set_properties_after_merging` should creata an `is_merged` property to track units that have been merged. This works if the new unit ids are not in the original set of units, but fail with the `take_first` naming strategy. Potentially similar issues are there with `set_properties_after_splitting` 