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.

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

package-source-builder/overlay/usr/bin/apertis-pkg-merge-updates

@@ -111,7 +111,7 @@ def main():
       print("Downstream is already up to date nothing to do")
       return
   try:
-    git('merge', args.upstream, '-m', f'Merge updates from {args.upstream}', '--stat', _fg=True)
+    git('merge', args.upstream, '--no-ff', '-m', f'Merge updates from {args.upstream}', '--stat', _fg=True)
   except Exception as e:
     print('Merge failed:')
     print(e)