evolve: fix divergence resolution when not merging a descendant In divergence resolution, when we merge other cset with divergent one we pass a `mergeancestor` arg to merge.update() and before this patch we were passing `mergeancestor` as True in every case. But it should be True only when we are merging a descendant onto an ancestor. When mergeancestor is True it does two things: 1) allows the merge if the destination is the same as the parent of the ctx (so we can use graft to copy commits) 2) informs update that the incoming changes are newer than the destination so it doesn't prompt about "remote changed foo which local deleted". So this patch change it to pass `mergeancestor` as True only when it is required. And changes in test file shows that it wasn't prompting either in those cases when it should (acc. to 2nd point) Test written by Pierre-Yves David, based the one updated in 5dbaabfe2c59.

test: fix a phase divergence test case to actually not contains conflict The previous test case had a conflict, but that conflict is currently not raise by phase divergence resolution. This is an independent bug to solve.

tests: use current instability names everywhere

test: apply unstability resolution to `test-unstability-resolution-result.t` The test cases in this file probably deserved to be dispatched in other more thematic test file.

test: rename and clarify content of test-unstable-content-divergent.t The troubles → instabilities renaming was not applied here.

test: rename and clarify content of test-unstable-orphan.t The troubles → instabilities renaming was not applied here.

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.