Only show text wrap and diff notes for text in merge requests.

Before this PR those options also showed for binary files for which they don't apply:

![screenshot from 2014-09-29 14 46 31 wrap text diff comments binary](https://cloud.githubusercontent.com/assets/1429315/4441810/5c39411a-47d7-11e4-84ff-38f567a21ecf.png)

Now they only show for text files.