While studying the patch "[PATCH v21 08/12] Allow non-btree indexes to
participate in mergejoin", I figured we could do the sortsupport.c piece
much simpler. The underlying issue there is similar to commits
0d2aa4d4937 and c594f1ad2ba: We're just using the btree strategy numbers
to pass information about the sort direction. We can do this simpler by
just passing a bool. See attached patch.