Draft: PoC for rebase-merged merge request diffs

What does this MR do and why?

PoC for rebase-merged merge request diffs

When MRs are rebase-merged, their commit history is rewritten and commits can no longer be found from the latest merge request diff.

In this PoC, I overwrote the merge head diff with the rebased commits, just to demonstrate its viability. It only works for merge trains because I don't know how to find the pre-merge target branch SHA for other types of merges.

In order to trigger the update that can then be found by the changelog, you need to run

MergeRequests::ReloadMergeHeadDiffService.new(mr).execute

The actual solution should probably create a new merge request diff type, e.g. merged, that is only present for merged MRs and contains the entire range of rebased commits.

See #436943

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