Junio C Hamano [off-list ref] writes:
Note that this still does not work very well for --boundary case
(see the last test added to t6016).
...
+test_expect_failure 'multi-root does not emit unnecessary post-root gap' '
+ git log --oneline --graph merge210~1...merge210~1^2~2 >actual &&
+ ! grep "^$" actual
+'
Obviously, this needs to be
git log --oneline --graph --boundary merge210~1...merge210~1^2~2 >actual &&
for it to fail.