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 #194813
checks-py2

00:01:11

passed #194814
checks-py3

00:01:04

passed #194819
doc

00:00:29

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

00:07:07

passed #194816
tests-py2-pure

00:04:00

passed #194817
tests-py3-cext

00:04:13

passed #194818
tests-py3-pure

00:06:56