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): ![Screenshot_2023-10-05_at_12.46.51_PM](/uploads/99deecd65c6e8776fb4a51994a1ab4e0/Screenshot_2023-10-05_at_12.46.51_PM.png) ![Screenshot_2023-10-05_at_12.47.03_PM](/uploads/ab03a4a35fde11e6080602677245cbec/Screenshot_2023-10-05_at_12.47.03_PM.png) ## Proposed To be discussed.
issue