`dolt_diff()` cannot accept a subquery argument

Mostly an inconvenience but good to fix.

```sql
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
```

Start by running the provided SELECT against dolt_diff() and inspect the implementation or entry point that evaluates its arguments, focusing on why subquery results are reported as uninitialized. Done means both commit_hash subqueries in the reproduction execute successfully and return the expected diff for myTable.

Written by the indexing model from the issue text.