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.
16 jobs for topic/default/fix-precheck in 53 minutes and 56 seconds (queued for 5 seconds)