message: Provide a helpful message how to start creating coverage.
  If the build has not been done yet, print out what to do.
diff: PROJECT_ROOT=$(dirname $0)/../../
  OUTPUT_DIR=coverage-html
  +COVERAGE_DATA=bazel-out/_coverage/_coverage_report.dat
  cd $PROJECT_ROOT
  -genhtml -o ${OUTPUT_DIR} bazel-out/_coverage/_coverage_report.dat
  +if [ ! -r "${COVERAGE_DATA}" ]; then
  + echo "First, run coverage tests; invoke"
  + echo "MODE=coverage .github/bin/build-and-test.sh"
  + exit
  +fi
  +
  +genhtml -o ${OUTPUT_DIR} ${COVERAGE_DATA}
  echo "Output in $(realpath ${OUTPUT_DIR}/index.html)"
  