Also show diff tab while editing blob with preview [failure unrelated]
Created by: cirosantilli
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.
The diff is equally useful whether there is preview or not.
