thoughtbot/guides — pull request #167 by jferris (2014-05-08 17:42:31+00:00)
Title: Rule for trailing commas in multi-line lists

Without a trailing comma on the last item in a list, adding a new item causes
a deletion and two insertion in Git diffs.

With a trailing comma on the last item in a list, adding a new item causes
only one insertion and no deletions.