pkg-merge-upstream-to-downstreams: Create a MR on conflicts

When the merge fails, really create a MR where the proposed-updates
branch still points to the from the unmerged upstream commit.

This was the intention all the time, to make failed merge visible in the
list of open MRs and provide a place where the conflicts should be
merged to developers.

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