Push option `merge_request.create` does not work for forks

After forking and changing a project, we usually want to create a merge request in the parent of the fork. While the push option merge_request.create usually works for branches, it does not for forks.

The merge request API is technically able to do so: https://docs.gitlab.com/ee/api/merge_requests.html#create-mr So it would be nice if we were able to pass an option merge_request.target_project_id or something similar.