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)
latest
Status Job ID Name Coverage
  Tests
manual check-pytype-py3 #230966
allowed to fail manual
failed checks-py2 #230953

00:04:29

failed checks-py3 #230954

00:03:36

passed rust-cargo-test-py2 #230955

00:02:23

passed rust-cargo-test-py3 #230956

00:02:20

passed test-py2 #230957

00:32:26

failed test-py2-chg #230964

00:53:53

passed test-py2-pure #230959

00:19:16

passed test-py2-rust #230961

00:35:17

passed test-py3 #230958

00:14:03

passed test-py3-chg #230965

00:08:16

passed test-py3-pure #230960

00:10:08

passed test-py3-rhg #230963

00:16:39

passed test-py3-rust #230962

00:15:58

manual windows-py3 #230967
windows allowed to fail manual
 
  Phabricator
skipped phabricator-refresh #230968
 
Name Stage Failure
failed
test-py2-chg Tests
Skipped test-symlinks.t#dirstate-v2: missing feature: Using the Rust extensions
Skipped test-update-atomic.t: system supports running with chg
Skipped test-verify-repo-operations.py: missing feature: allow slow tests (use --allow-slow-tests)
Skipped test-wireproto-command-capabilities.t: system supports running with chg
Failed test-subrepo-git.t: timed out
# Ran 889 tests, 95 skipped, 1 failed.
python hash seed: 1983208415
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
checks-py2 Tests
  0.016  39.186  48.430  21.150  39.169   test-check-module-imports.t
0.019 43.618 68.580 22.010 43.599 test-check-config.t
0.086 45.604 265.700 25.880 45.518 test-check-format.t
0.019 59.626 304.650 27.010 59.607 test-check-pyflakes.t
0.001 67.491 353.240 28.250 67.490 test-check-code.t
0.067 110.657 716.320 50.380 110.590 test-check-cargo-lock.t
0.062 114.764 799.290 52.320 114.702 test-check-pylint.t
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
checks-py3 Tests
  0.041  12.636 252.330   8.010  12.595   test-check-format.t
0.008 13.500 265.220 8.170 13.492 test-check-config.t
0.003 14.569 279.160 8.500 14.566 test-check-module-imports.t
0.009 29.522 307.760 8.870 29.513 test-check-pyflakes.t
0.001 35.226 342.050 9.410 35.225 test-check-code.t
0.018 57.909 397.800 10.440 57.891 test-check-pylint.t
0.027 58.971 693.880 29.520 58.944 test-check-cargo-lock.t
Cleaning up file based variables
ERROR: Job failed: exit code 1