Use patch-id of the merge request diff as cache key for diff

Background

While rolling out #388778, it was found out that the cache is getting invalidated often because it's using the diff ID of the HEAD diff which changes whenever target branch changes.

Proposal

Use the patch-id of the merge request diff instead of the diff ID as the cache key. This way, when the diff didn't really change even if the target branch changes, the cache won't get invalidated.