repo sync: Use the new merge-upstream API if available #4942
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This helps avoid the errors when a new commit from upstream is not yet available in the fork, resulting in a HTTP 404 when trying to update the ref in the fork.
If the merge-upstream API is unavailable (404) or errors out (409/422), the functionality falls back to the previous functionality of manually updating refs. This ensures that
--forcestill has effect.Fixes #4907