precheck: fix false warning about divergence creation, in hg prune

In case of pruning with no successors, there is no chance for new divergence.
It was happening becuase current precheck logic assumes that every revision
being rewritten will be replaced with at least one successor.

Changes in test file demonstrate the fixed bug.
8 jobs for topic/default/fix-prune in 16 minutes and 49 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Test
passed checks-py2 #254168

00:02:35

passed checks-py3 #254169

00:02:21

passed doc #254174

00:01:41

manual pytype #254167
allowed to fail manual
failed tests-py2-cext #254170

00:13:47

failed tests-py2-pure #254171

00:15:15

failed tests-py3-cext #254172

00:15:23

failed tests-py3-pure #254173

00:16:47

 
Name Stage Failure
failed
tests-py3-pure Test
Failed test-tutorial.t: output changed
Failed test-uncommit-interactive.t: output changed
Failed test-unstable-orphan.t: output changed
Failed test-userguide.t: output changed
# Ran 217 tests, 8 skipped, 46 failed.
python hash seed: 3856897546
Cleaning up file based variables
nothing to do
ERROR: Job failed: exit code 1
failed
tests-py3-cext Test
Failed test-tutorial.t: output changed
Failed test-uncommit-interactive.t: output changed
Failed test-unstable-orphan.t: output changed
Failed test-userguide.t: output changed
# Ran 217 tests, 8 skipped, 46 failed.
python hash seed: 296114728
Cleaning up file based variables
nothing to do
ERROR: Job failed: exit code 1
failed
tests-py2-pure Test
Failed test-tutorial.t: output changed
Failed test-uncommit-interactive.t: output changed
Failed test-unstable-orphan.t: output changed
Failed test-userguide.t: output changed
# Ran 217 tests, 8 skipped, 46 failed.
python hash seed: 2366923292
Cleaning up file based variables
nothing to do
ERROR: Job failed: exit code 1
failed
tests-py2-cext Test
Failed test-tutorial.t: output changed
Failed test-uncommit-interactive.t: output changed
Failed test-unstable-orphan.t: output changed
Failed test-userguide.t: output changed
# Ran 217 tests, 8 skipped, 46 failed.
python hash seed: 3098025105
Cleaning up file based variables
nothing to do
ERROR: Job failed: exit code 1