`dolt_diff()` cannot accept a subquery argument

[zachmu]

Mostly an inconvenience but good to fix.

SELECT to_id FROM DOLT_DIFF(                                                                              
    (SELECT commit_hash FROM dolt.log ORDER BY date DESC OFFSET 1 LIMIT 1),                               
    (SELECT commit_hash FROM dolt.log ORDER BY date DESC LIMIT 1),                                        
    'myTable');                                                                                              

ERROR: attempted to evaluate uninitialized subquery