code: function getStagedFiles(callback) {
  
      const options = [ 'diff', '--cached', '--name-only', '--diff-filter=ACM' ]
  
      execFile('git', options, (err, stdout) => {
  
          if (err) return callback(err)
  
          const stagedFiles = stdout
              .split('\n')
              .filter(filename => filename.match(/.js$/))
  
          callback(null, stagedFiles)
  
      })
  
  }
docs: Calls callback with list of staged files
  @param {Function} callback
  @returns {void}