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

I ran Hound locally and enabled in for one of my repos to test these changes.

1. Opened a PR with one violation, which Hound caught:
![screen shot 2014-07-11 at 3 37 09 pm](https://cloud.githubusercontent.com/assets/1855523/3559562/4ac8ae08-094c-11e4-9818-37374deefe7e.png)

2. Amended my original commit without changing any code and force pushed. Hound made no additional comments:
![screen shot 2014-07-11 at 3 38 39 pm](https://cloud.githubusercontent.com/assets/1855523/3559589/b838685c-094c-11e4-8937-76779cd0f8a2.png)


Good idea. Done: https://github.com/thoughtbot/hound/commit/9dd13c4cc62715e2002180dac03da248247d2259