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.
issue