pkg-merge-upstream-to-downstreams: Fix targeting stable branches
requested to merge wip/em/pkg-merge-upstream-to-downstreams-fix-stable-branches into apertis/v2022dev0
When targeting development/preview releases the script to merge changes
from upstream works fine, but when aiming at stable releases the part
that tries to detect which branch to target (-security vs. -updates)
fails to find the refs it looks for because it searches the local refs
instead of the remote ones.
Point it to the refs under the origin remote to make it happy.
See https://gitlab.apertis.org/pkg/target/linux/-/jobs/488950 for a job that fails to find the right refs.
The https://gitlab.apertis.org/pkg/target/linux/-/jobs/490576 jobs instead builds on https://gitlab.apertis.org/pkg/target/linux/-/merge_requests/59 to exercise this patch.