panic for dolt_diff tables when in detached head mode

[zachmu]

For database named test, insert some rows into a table named a, then

set @@test_head=commit('-m', 'test');
select * from dolt_diff_a;
panic in iterPartitions: runtime error: invalid memory address or nil pointer dereference

[timsehn]

It does look like detached head mode is deprecated:

mysql> set @@test_head=commit('-m', 'test');
ERROR 1105 (HY000): Variable 'test_head' is a read only variable