Change how check_modified_files.sh works #20 [skip modfiles]

Checking merged master version instead of HEAD in check_modified_files script.

This means that when modifying files in a MR which has not yet been merged we won't need to specify [skip modfiles]

Closes #20 (closed)

Checks:

• Modify existing file -> commit without [skip modfiles] -> CI should fail https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/jobs/6104486
• Add file in existing directory -> commit without [skip modfiles] -> CI should fail https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/jobs/6104493
• Modify file in unmerged branch -> commit without [skip modfiles] -> CI should succeed https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/jobs/6104498 Note that before merging this it will not be possible for the CI to succeed since scripts/check_modified_files.sh has been modified from master, however what I tested here was that I added a new DSID 950007 and then modified the jO inside there. You can see that the modification of the jO is not picked up by the script since 950007 does not yet exist in the master.