We were incorrectly setting `moved_before` as `false` after checking
for unique constraint violation, but the reality is that after the
uniqueness check, we are already correctly positioned -- if no match
was found, the cursor is positioned at either:
a) no row, or
b) at the first entry that is greater than the key we are inserting.
This means we don't have to move anymore and can just insert.
bdbd021bbb