obsolescence: add test for the "branch replacement" logic during push, case A2 Mercurial checks for the introduction of new heads on push. Evolution comes into play to detect if existing branches on the server are being replaced by some of the new one we push. The current code for this logic is very basic (eg: issue4354) and was poorly tested. We have a better implementation coming in the evolve extension fixing these issues and with more serious tests coverage. In the process of upstreaming this improved logic, we start with adding the test case that are already passing with the current implementation. Once they are all in, we'll upstream the better implementation and the extra test case. See inline documentation for details about the test case added in this changeset.