pkg-merge-updates: Always produce a merge commit

Avoid fast forward merges, which may happen in specific corner cases, like a downstream mirroring a repository and making their downstream/v2021 branch initially point to apertis/v2021: in that case a pull from apertis/v2021 would produce a fast-forward and we end up amending the HEAD of apertis/v2021 to append the downstream suffix, rather than amending the merge commit.