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.
Differential Revision: https://phab.mercurial-scm.org/D12002
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[10]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate