Also show diff tab while editing blob with preview

Before this PR, you cannot see the diff if the blob you are editing has a preview, like Markdown: the Diff button would be replaced with preview instead.

Now you can: both buttons have been split up. Diff always shows, and Preview only shows if there is a preview available.

![screenshot from 2014-10-05 13 47 21 diff preview after](https://cloud.githubusercontent.com/assets/1429315/4518354/e3e76eae-4c85-11e4-9375-eead5cb6e36b.png)
 
![screenshot from 2014-10-05 13 51 20 edit preview diff](https://cloud.githubusercontent.com/assets/1429315/4518355/fb057fcc-4c85-11e4-9745-f983045b74a4.png)