Avoid window functions in MR diff preloader

What does this MR do?

This MR slightly alters the MR diff preloader query to avoid window functions. Looks like using window functions + distinct might cause too many rows to be read. This problem came up with a similar preloading attempt: #329621

• Old query: https://explain.depesz.com/s/TNPr
• New query: https://explain.depesz.com/s/LcO1

Note: there is no significant performance difference here. I want to use this preloader as an example for has_one preloading where the association definition contains an ORDER BY clause.

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