Fix erroneous logic to disallow duplicate comments

* This commit fixes two logic errors in be2dd34178
* In `Commenter#previous_line_comments`, compare position of existing
  comment to line patch position rather than line number
* In `CommentPolicy#violation_not_previously_reported?`, compare new
  violation message to body content of existing comments rather than the comment
  objects
* https://trello.com/c/ha2hgFj2/283-avoid-commenting-a-second-time-after-a-force-push