I basically just added an if-else block here so it won't throw an error if there are no migration differences

db_rollback_to_common.rake

#
# Usage:
# rake db:rollback_to_common branch=production
# git checkout -m production
# rake db:migrate
#
namespace :db do
  desc 'Rolls back database to common migration state between current branch and another'
  task :rollback_to_common do
    FileUtils.cd(RAILS_ROOT) do
      diff = `git diff --name-only HEAD #{ENV['branch']} db/migrate`
      first_difference = diff.split("\n").sort.first
      migration_files = Dir.glob(File.join('db', 'migrate', '*.rb')).sort
      first_difference_index = migration_files.index(first_difference)
      if first_difference_index == nil
        puts "No migration difference between this branch and #{ENV['branch']}"
      else
        common_version_index = first_difference_index - 1
        version = migration_files[common_version_index].match(/^db\/migrate\/(\d{14})/)[1]
        puts `rake db:migrate VERSION=#{version}`
      end
    end
  end
end