code: function addModifiedFile (info, filename) {
    if (info.modifiedFiles.indexOf(filename) === -1) {
      info.modifiedFiles.push(filename)
    }
  }
docs: Adds the given filename to info.modifiedFiles if it doesn't already exist
  @param {PrInfo} info - the info for the PR
  @param {String[]} info.modifiedFiles - the list of modified files so far
  @param {String} filename - the filename to add to info.modifiedFiles if it's not already there