Fix commit comments on first line of diff not rendering in Merge Request Discussion view.

Example can be seen near the bottom on this MR: !1533.

Before:

![Screen Shot 2015-02-20 at 10.24.34](https://dev.gitlab.org/uploads/gitlab/gitlabhq/35600b98b5/Screen_Shot_2015-02-20_at_10.24.34.png)

After: the [note](https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1533/diffs#note_36449) is actually rendered.

See merge request !1552