change default branch name for core in test upstream script
MODIFY
ci/test-asciidoctor-upstream.sh
ci/test-asciidoctor-upstream.sh
set -euo pipefail
# to build against a tag, set REF to a git tag name (e.g., refs/tags/v1.5.2)
-REF=${1:-refs/heads/master}
+REF=${1:-refs/heads/main}
if [[ $REF != refs/* ]]; then
REF=refs/heads/$REF
fi

change default branch name for core in test upstream script