evolve: update some conditionals

Now as we know that, in public-content-divergence cset in public
phase will always be the `divergent` one. We don't need to check
`if not other.mutable()`.
8 jobs for topic/default/public-content-div in 5 minutes and 28 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #180092
checks-py2

00:03:18

passed #180093
checks-py3

00:01:13

passed #180098
doc

00:00:39

manual #180091
allowed to fail manual
pytype
passed #180094
tests-py2-cext

00:05:28

passed #180095
tests-py2-pure

00:05:06

passed #180096
tests-py3-cext

00:03:07

passed #180097
tests-py3-pure

00:04:55