pkg-merge: Always submit a new 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.

Task: T7797

package-source-builder/overlay/usr/bin/apertis-pkg-merge-upstream-to-downstreams

@@ -110,6 +110,10 @@ def main():
       push_this_merge_request()
       continue
 
+    print(f"Submit {proposed_branch} for merging into {downstream_branch} the updates from {upstream_branch}")
+    git("branch", proposed_branch, upstream_branch)
+    push_this_merge_request()
+
     print(f"Attempt merging {upstream_branch} into {downstream_branch} via {proposed_branch}")
     git("checkout", "--force", "-B", "tmp", downstream_branch)
     apertis_pkg_merge_updates(f"--downstream={downstream_branch}", f"--upstream={upstream_branch}", f"--local-version-suffix={local_suffix}", _fg=True)