feat: add GITLAB_OA_OLDREV environment variable for merge requests

[julian-piehl]

Adds the environment variable GITLAB_OA_OLDREV to Merge Request Pipelines.

Allows users to check if there are code changes using their own pipeline and not the plugin settings.

Testing done

Submitter checklist

• [X] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [X] Ensure that the pull request title represents the desired changelog entry
• [X] Please describe what you did
• [ ] Link to relevant issues in GitHub or Jira
• [ ] Link to relevant pull requests, esp. upstream and downstream changes
• [X] Ensure you have provided tests - that demonstrates feature works or fixes the issue