precheck: fix false warning about content-divergence creation

Before this patch, if we try to `hg prune` (without any successors) an
already obsoleted cset which has at least one successor, it would false
warn about new content-divergence. As we know pruning cset without any
successors can not create any divergence.

--HG--
branch : stable
17 jobs for topic/stable/fix-precheck in 59 minutes and 9 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Tests
passed check-pytype-py3 #254023

00:25:33

passed checks-py2 #254010

00:05:34

passed checks-py3 #254011

00:05:06

passed rust-cargo-test-py2 #254012

00:04:19

passed rust-cargo-test-py3 #254013

00:04:17

passed test-py2 #254014

00:27:44

passed test-py2-chg #254021

00:56:29

passed test-py2-pure #254016

00:28:23

passed test-py2-rust #254018

00:30:08

passed test-py3 #254015

00:26:51

passed test-py3-chg #254022

00:08:24

passed test-py3-pure #254017

00:13:30

passed test-py3-rhg #254020

00:15:47

passed test-py3-rust #254019

00:10:47

manual windows-py3 #254024
windows allowed to fail manual
manual windows-py3-pyox #254025
windows allowed to fail manual
 
  Phabricator
passed phabricator-refresh #254026

00:02:39