pkg-merge-upstream-to-downstreams: Check $RELEASE-updates branches

When pulling from Apertis to downstreams we need to also check branches
like `apertis/v2021-updates` when looking for updates to import.

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

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

@@ -30,6 +30,7 @@ def ensure_branch(name: str, fallbacks: list) -> str:
 
 def existing_upstream_branches(upstream: str):
     branches = set((
+        f'{upstream}-updates',
         f'{upstream}-proposed-updates',
         f'{upstream}-security',
         f'{upstream}',