compat: make compatible with upstream change to put nullid on repo

This patch makes evolve compatible across Mercurial core commit
d55b71393907.
8 jobs for topic/default/compat in 7 minutes and 7 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed checks-py2 #194813

00:01:11

passed checks-py3 #194814

00:01:04

passed doc #194819

00:00:29

manual pytype #194812
allowed to fail manual
passed tests-py2-cext #194815

00:07:07

passed tests-py2-pure #194816

00:04:00

passed tests-py3-cext #194817

00:04:13

passed tests-py3-pure #194818

00:06:56