evolve: fix bug in public content divergence merging logic in continue case

For continue case, we were not updating the `divergent` due to which
_mergecontentdivergents() was not getting the right cset during the
merge of two divergent csets instead it was getting the old cset
which needed to be rebased.

Changes in test file reflect the fixed behavior.
8 jobs for topic/default/khanchi-bugfix-public-divergence in 7 minutes and 52 seconds (queued for 1 minute and 42 seconds)
Status Name Job ID Coverage
  Test
passed checks-py2 #92896

00:01:13

passed checks-py3 #92897

00:02:44

passed doc #92902

00:00:24

manual pytype #92895
allowed to fail manual
passed tests-py2-cext #92898

00:07:25

passed tests-py2-pure #92899

00:04:57

passed tests-py3-cext #92900

00:03:43

passed tests-py3-pure #92901

00:03:20