tests: add test showing that rebase of extinct commit with successor fails As the test case shows, attempting to rebase a commit that has a successor that is not in the rebase set and not in the destination currently fails because it "will cause divergences". However, it doesn't seem like there's any harm in skipping the extinct commit. I suspect this case missed simply because extinct revisions are usually hidden. Differential Revision: https://phab.mercurial-scm.org/D4407