Outdated diff refers to a wrong parent commit

Summary

An outdated diff for a merge request becomes linked to a totally different parent commit

Steps to reproduce

N/A

Example Project

A buggy diff from GitLab EE MR

What is the current bug behavior?

1. The diff contains a huge set of changes which is wrong.
2. The link View file @ba6c3ac8 leads to the commit with different SHA which appears to be an automated merge from CE to EE.
3. The wrong diff is visible in the discussion note preview.

What is the expected correct behavior?

The expected behavior: the diff between parent and child commit shows the correct set of changes

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

Possible fixes

N/A