count(DINSTINCT) with a large number of distinct values runs forever

When attempting test200.sh, the test never finished.
The query

select count(distinct c1) as count2 from test200 where c1 <= 139000000;

never returns. It takes around 3 minutes in 5.6.1.
A little investigation shows an internal deadlock in fifo.h.

Additionally, there appears to be a slowdown in lesser counts:

select count(distinct c1) as count2 from test200 where c1 <= 13900000;

takes almost 3 minutes in 6.1.1, but 15 seconds in 5.6.1.