evolve: preserve date when resolving content-divergence (issue6202)

This patch changes the implementation to do a 3 way merge. In case of actual
conflict, it take the newest date from both sides of the content-divergence.

--HG--
branch : stable
5 jobs for topic/stable/issue6202 in 4 minutes and 26 seconds (queued for 3 minutes and 54 seconds)
Status Name Job ID Coverage
  Test
passed flake8 #51205

00:00:25

passed tests-py2-cext #51206

00:04:20

passed tests-py2-pure #51207

00:04:20

passed tests-py3-cext #51208

00:04:02

passed tests-py3-pure #51209

00:03:50