Provide context when viewing commits in a merge request

Problem

The "commits" tab in a merge request presents a flat list of commits associated with the merge requests. Once user clicks on a particular commit, GitLab takes them to the "changes" tab of the MR, losing context of the other commits and how the commit being viewed relates to the others.

Further info

Eric describes the problem in the initial 40 sec of this video zoom_0

Proposal

When clicking on a particular commit from the "commits" tab, we should:

1. Present which commit out of how many is currently being viewed
2. Provide a clear option to navigate back to the list of commits