If a BAT head is sorted in the opposite direction than requested, stable sort (ssort) cannot use BATrevert, unless the head is unique (key).
If a BAT head is sorted in the opposite direction than requested,
stable sort (ssort) cannot use BATrevert, unless the head is unique (key).

This fixes a but that was detected by test tests/from_Twente/moa/Tests/test.milM,
which has been failing since sort_ht() uses ssort().

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb