Creating a merge request from a partially merged (squashed) branch shows a common ancestor diff instead of a HEAD diff

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It is a known issue that GitLab shows a common ancestor diff in the MR preview.

However, when an MR has already been created, the diff should show only the relevant changes. This is not the case when the branch has only partially been merged:

1. Create a diverged branch from the main
2. Merge (squash!) only some changes from the branch by editing some commits
3. Create a new MR from the branch to the main and observe the same common ancestor diff in the MR's preview and in the MR's body when it's created.

Related:

• #371018 (closed)
• #416700