tests: update divergence test for `hg fix` to actually result in divergence We have a test that checks that `hg fix` errors out if it might cause divergence. However, the test simply prunes the commit it then tries to fix, so fixing it wouldn't actually cause divergence. That works because the implementation is simple enough that it doesn't notice the difference. I'm about to make the implementation smarter, so let's fix the test first. Differential Revision: https://phab.mercurial-scm.org/D10267