This script extracts the first line from stdin, then escapes single quotes, so it can be safely passed as an argument.

escape_single_quotes_from_first_line.sh

# This script extracts the first line from stdin,
# then escapes single quotes, so it can be safely passed as an argument.

# get first line from stdin
FIRST_LINE=$(cat - | head -1)

# escape single quotes
echo "${FIRST_LINE}" | sed -e "s/'/'\\\\''/g"

test.sh

./escape_single_quotes_from_first_line.sh << EOF
'first' "line"
second line
third line
EOF
# => should return: '\''first'\'' "line"

usage-in-github-ci.yml

jobs:
  dummy:
    steps:
      - name: Get Commit Message
        run: |
          FIRST_LINE=$(git log --format=%B -n 1 ${{ github.event.after }} | ./escape_single_quotes_from_first_line.sh)
          echo "message=${FIRST_LINE}" >> $GITHUB_OUTPUT
        id: extract_message

      - name: Pass To Mycommand
        run: ./mycommand --message '${{ steps.extract_message.outputs.message }}'