Fix total_tuple_count for BackfillMergeRequestFileDiffsPartitionedTable

What does this MR do and why?

Fixes total_tuple_count for BackfillMergeRequestFileDiffsPartitionedTable.

When this batched background migration was enqueued, we did so without populating total_tuple_count since the usual helper method isn't compatible with cursor batched background migrations.

This uses the usual means of estimating total_tuple_count from pg_class and populates the missing value in an existing batched_background_migrations row if it exists.

References

• Backfill data into merge_request_diff_files (#541972 - closed)

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