Changes tab does not show merge conflict in diff (invalid cache key for diffs_batch)

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We don't update the cache key for the diffs_batch.json endpoint for Merge Requests when diff has conflicts. This causes for the 'Changes' tab to show irrelevant results without the conflicts.

To reproduce:

1. Create a branch with changes
2. Create a merge requests for that branch
3. Open the 'Changes' tab, observe changes
4. Change the same lines in the target branch
5. Refresh the 'Changes' tab
6. Observe no conflicts in the diff
7. Observe diffs_batch.json response coming from disk cache, not from the server