Update for compatibility with Trixie

Update for compatibility with Trixie:

• apertis-pkg-merge-upstream-to-downstreams: Don't print useless information while canceling pipelines.
• apertis-pkg-merge-upstream-to-downstreams: ensure merge request is created against the right project.
• Make sure the returned value of an executed sh is still a RunningCommand object.

tools/apertis-pkg-merge-updates

@@ -176,6 +176,7 @@ def main():
                 "merge",
                 "--ff-only",
                 rebase_tip,
+                _return_cmd=True,
                 _ok_code=[0, 128],
                 _out="/dev/stdout",
                 _err="/dev/stderr",
@@ -215,6 +216,7 @@ def main():
         args.upstream,
         ":!debian/changelog",
         ":!debian/apertis/*",
+        _return_cmd=True,
         _ok_code=[0, 1],
     )
     msg = [f"Sync from {args.upstream}."]