apertis-pkg-merge-upstream-to-downstreams: ensure merge request is created against the right project

In case of a forked project, like we are doing with the test of the rebase,
the GitLab API creates by default the MR against the original project
instead of the forked project.
To ensure the merge request is created against the forked project, we use
the push option merge_request.target_project to overwrite the default
behavior and to create the merge request against the forked project.

See:
https://docs.gitlab.com/ee/topics/git/commit.html#push-options-for-merge-requests



Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>