The script to check this was:

  for file in "$@"
  do
    echo "Trying $file"
    sed -i "s/^require .spec_helper./require 'fast_spec_helper'/" "$file"
    bin/rspec --order defined --format progress "$file" || git co "$file"
  done

Note, some order dependent specs were excluded.

Verified via:

  while :; do parallel_rspec $(git diff HEAD~ --name-only | xargs) || break; done