pkg-merge-upstream-to-downstreams: Always create MR, even with conflicts

Create a MR even when the local attempt to merge failed. In that case
let the proposed updates branch point to the upstream branch and create
a MR to submit it to the downstream branch.

This makes the failed updates more visbile and easier to track than the
failed pipelines.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>