List new commits for newly pushed branch in activity view.

When someone pushes commits, I want to see them, regardless if it's a new or existing branch. If it's a new branch, we compare with the default branch (usually master).

![Screen_Shot_2015-03-17_at_15.49.13](https://dev.gitlab.org/gitlab/gitlabhq/uploads/4cb7e0d833bc83a30197db81d4b273bc/Screen_Shot_2015-03-17_at_15.49.13.png)

See merge request !1711