fix: resolve pr head branch on merge-queue

[Lars Kemper (larskemper)]

Problem

Since #163, branch sync ignores matching downstream branches in the queue.
Coupled upstream + downstream branches failing in the queue.

get_ref() detects the queue but no longer resolves the head branch:

echo "Merge queue detected. Using REF"
return                      # REF stays = gh-readonly-queue/... (only exists upstream)

That ref never exists downstream, so step 1 always misses and falls back to trunk.
It's invisible for prs without a matching downstream branch.

#163 "queue stays open while downstream runs" is about timing and still holds. this
only re adds which branch to check out, a separate concern.

upstream: shopware/shopware#17306
downstream: https://github.com/shopware/SwagCommercial/pull/2907
faling commercial run: https://github.com/shopware/SwagCommercial/actions/runs/27616756826

[Marcel Kräml (mkraeml)]

This will introduce the old behavior again 🙈
I think we need to extend the check loop to also check the head branch if it doesn't find the merge queue

[Marcel Kräml (mkraeml)]

I've changed the code to also resolve the HEAD_REF and test this if the REF is not found.
We changed the get_ref code to be able to merge external contributions.