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 #152500
checks-py2

00:01:06

passed #152501
checks-py3

00:03:00

passed #152506
doc

00:00:21

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

00:04:09

passed #152503
tests-py2-pure

00:03:36

passed #152504
tests-py3-cext

00:04:47

passed #152505
tests-py3-pure

00:03:35