Run generate_stacks only in non-master branches and only for changed files

.gitlab-ci.yml

@@ -48,12 +48,15 @@ pytest:
 generate_stacks:
   extends: .setup_environment
   stage: run
+  rules:
+    - if: '$CI_COMMIT_BRANCH != "master"'
   artifacts:
     paths:
       - to_release.json
     expire_in: 1 week
   script:
-    - lb-stacks-to-release /cvmfs/lhcb.cern.ch/lib/ ./data/stacks | tee to_release.json
+    - git fetch origin master
+    - lb-stacks-to-release /cvmfs/lhcb.cern.ch/lib/ $(git diff --name-only --diff-filter=ACRM FETCH_HEAD HEAD | grep "^data/stacks/.*\.yml") | tee to_release.json
 
 generate_project_list:
   extends: .setup_environment