On the first load, the merge request tabs would be requested in JSON format to render
the HTML. This would cause the "Reload with full diff" button to create a link to diff.json.

Closes #2041

These have been replaced by the "requiresInput" behavior.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

MergeRequest#show performance improvements

This is a first pass on improving the performance of the `MergeRequests#show` page. Notable changes:

- The "Commits" tab is loaded lazily, so the initial page load should be much faster for MRs with many commits.
- Relative timestamps via `timeago` are only initialized once per load instead of `O(n^2)`. This greatly improves frontend rendering times for a large number of commits.
- Refactored `User.find_for_commit` to use a single ARel-generated SQL query instead of the old method which resulted in one query, and could result in up to three.

See merge request !838