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

tests/test-fix.t

@@ -1106,14 +1106,13 b' obsolete revision.'
 
   $ printf "foo\n" > foo.changed
   $ hg commit -Aqm "foo"
-  $ hg debugobsolete `hg parents --template '{node}'`
-  1 new obsolescence markers
-  obsoleted 1 changesets
+  $ hg ci --amend -m rewritten
   $ hg --hidden fix -r 0
   abort: fixing obsolete revision could cause divergence
   [255]
 
   $ hg --hidden fix -r 0 --config experimental.evolution.allowdivergence=true
+  2 new content-divergent changesets
   $ hg cat -r tip foo.changed
   FOO