Refactor MergeRequest#merge_request_diff to have clearer intention

Background

During an async debugging session of a seemingly unrelated issue in Slack, we found out that MergeRequest#merge_request_diff can actually result to a unpersisted MergeRequestDiff instance.

Here are some of the messages related to it (this is to keep the information since we delete Slack messages at some point):

Proposed

To be discussed.