Pre-load diff when viewing a MR

Description including problem, use cases, benefits, and/or goals

When I visit a merge request, chances are that I'm there to look at the diff.

Right now, I'll have to click on diff and then wait for the diff to load. This experience could be better if we start preloading the diff the moment you enter the MR.

Proposal

Not sure. Is this feasible to do @dzaporozhets @DouweM @jschatz1 @stanhu?

Links / references

Based on a feedback session with a customer.