code: function commonDir (files) {
    return files
      .map(path.dirname)
      .map(function (dir) {
        return dir.split(path.sep)
      })
      .reduce(commonSequence)
      .concat([''])
      .join(path.sep)
  }
docs: Returns the parent directory common to each path.
  @param {Array} files - An array of file paths to inspect
  @returns {string} A single path ending with the path separator, e.g. '/user/some/folder/'
  @alias module:common-dir
  @example
  > const commonDir = require('common-dir')
  > files = [
  '/Users/75lb/one/package.json',
  '/Users/75lb/one/test',
  '/Users/75lb/two/test/main.js'
  ]
  > commonDir(files)
  '/Users/75lb/'