[refactor] 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 occurrences.

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)