evolve: refactor the code which swap two nodes (in divergence resolution)
I see that we are using the code to swap the "divergent" and "other" node;
and updating the evolvestate accordingly at many places in divergence
resolution. This patch extract that code to a function.
Although I don't have strong opinion on this patch. It's a volunteered
patch to remove some redundancy.
test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t