Fix sync: allow unrelated histories for initial upstream merge

.forgejo/workflows/sync.yaml

@@ -23,7 +23,7 @@ jobs:
       - name: Check for changes
         id: check
         run: |
-          if git merge-base --is-ancestor upstream/master HEAD; then
+          if git merge-base --is-ancestor upstream/master HEAD 2>/dev/null; then
             echo "changed=false" >> "$GITHUB_OUTPUT"
           else
             echo "changed=true" >> "$GITHUB_OUTPUT"
@@ -32,7 +32,7 @@ jobs:
       - name: Merge and push
         if: steps.check.outputs.changed == 'true'
         run: |
-          git merge upstream/master --no-edit
+          git merge upstream/master --allow-unrelated-histories --no-edit -X theirs
           git push https://joshii:${{ secrets.REGISTRY_TOKEN }}@git.joshuahirsig.xyz/docker/worldguessr.git HEAD:main
 
       - name: Install Docker CLI