utility: deal with a parent becoming empty/pruned with a split grandparent

When the obsolete parent of a revision being evolved becomes empty and gets
pruned, it should look at the successors of the grandparent. If the grandparent
was split, however, we were not handling the split properly and were still trying
to find successors of the parent.
8 jobs for topic/default/evolve_multiprune in 4 minutes and 48 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed checks-py2 #152500

00:01:06

passed checks-py3 #152501

00:03:00

passed doc #152506

00:00:21

manual pytype #152499
allowed to fail manual
passed tests-py2-cext #152502

00:04:09

passed tests-py2-pure #152503

00:03:36

passed tests-py3-cext #152504

00:04:47

passed tests-py3-pure #152505

00:03:35