split: compatibility with cmdutil.revert() without parents argument

`assert` is here because I'm not sure if this is enough, and we might not have
tests that fail this specific assert.
8 jobs for topic/default/revert-parents in 3 minutes and 53 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed checks-py2 #91740

00:02:49

passed checks-py3 #91741

00:01:15

passed doc #91746

00:00:46

manual pytype #91739
allowed to fail manual
passed tests-py2-cext #91742

00:02:59

passed tests-py2-pure #91743

00:03:38

passed tests-py3-cext #91744

00:03:53

passed tests-py3-pure #91745

00:03:46