Modification check should also veto new files in existing DSID directories

As far as I understand, we currently only check for modified files in

https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/blob/master/scripts/check_modified_files.sh

But this should be extended to reject new files in existing DSID directories, since otherwise we would allow to add a common base fragment like Pythia8_i/Pythia8_..._Common.py to an existing directory and thus overwrite the one that was used from the release (thus changing the physics output).