Factor out commit list from compare and new MR.

The exact same piece of code was copied twice.

Besides those, there is also a third very similar copy which has started to diverge for the merge requests show view at: https://github.com/gitlabhq/gitlabhq/blob/174c00cf2c026a3bdc61d94b45195a5e5c99202f/app/views/projects/merge_requests/show/_commits.html.haml#L6 .  It has diverged as it now:

- has a list icon
- hides to 8 items on page load

I propose all of those 3 cases be made the same. If not, at least we should further refactor part of the MR show with the other 2.

Just to show clearly what is the object in question:

![screenshot from 2014-08-31 13 07 27 commit list](https://cloud.githubusercontent.com/assets/1429315/4101701/d996c5d6-30ff-11e4-850f-8c9c00a272d3.png)