changedFiles = (oldFileList, newFileList) => {
  const working = oldFileList.slice()
  return newFileList.filter((newFile) => {
    const found = working.findIndex((oldFile, i) => sameFile(oldFile, newFile))
    if (found < 0) {
      return true
    }
    working.splice(found, 1)
    return false
  })
}
Compare the contents of the old and the new file lists.
@see flattenFiles() below
@param {Array} oldFileList - the old list of files
@param {Array} newFileList - the new list of files
@return {Array} a list of the changed files