precheck: fix false warning about divergence creation during 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 7 minutes and 13 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed checks-py2 #231026

00:01:25

failed checks-py3 #231027

00:01:15

passed doc #231032

00:00:36

manual pytype #231025
allowed to fail manual
failed tests-py2-cext #231028

00:07:13

failed tests-py2-pure #231029

00:04:17

failed tests-py3-cext #231030

00:07:10

failed tests-py3-pure #231031

00:04:08

 
Name Stage Failure
failed
checks-py3 Test
+  [123]

ERROR: test-check-flake8.t output changed
!...
Failed test-check-flake8.t: output changed
# Ran 8 tests, 0 skipped, 1 failed.
python hash seed: 2152563042
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
tests-py2-cext Test
Failed test-touch.t: output changed
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 213 tests, 8 skipped, 46 failed.
python hash seed: 537393579
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
tests-py3-cext Test
Failed test-touch.t: output changed
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 213 tests, 8 skipped, 46 failed.
python hash seed: 2638004190
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
tests-py2-pure Test
Failed test-touch.t: output changed
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 213 tests, 8 skipped, 46 failed.
python hash seed: 2890187494
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
tests-py3-pure Test
Failed test-touch.t: output changed
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 213 tests, 8 skipped, 46 failed.
python hash seed: 58352486
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
checks-py2 Test
+  [123]

ERROR: test-check-flake8.t output changed
!..
Failed test-check-flake8.t: output changed
# Ran 8 tests, 0 skipped, 1 failed.
python hash seed: 2919945600
Cleaning up file based variables
ERROR: Job failed: exit code 1