Describe the feature you'd like to request

If you try to use the --filter=[origin/main] command to detect the changes between current and the origin/main. Only on Gitlab the default behaviour is to use headless branches for their pull/merge requests and appears not to work.

Describe the solution you'd like

I think it would be great to improve the wait the git diff command is executed it appears that the following works for Gitlab CI but I have to confirm it again:

git diff --name-only origin/$CI_MERGE_REQUEST_SOURCE_BRANCH_NAME origin/$CI_MERGE_REQUEST_TARGET_BRANCH_NAME

In the above $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME is the name of the pull request and the $CI_MERGE_REQUEST_TARGET_BRANCH_NAME would be the branch merging into typically main.

Describe alternatives you've considered

Stop using --filter=[origin/main] or similar filter arguments