tests: add flat manifest case in test-narrow-widen-non-ellipsis.t We had this test with tree manifest only because the tests were broken and we were fixing them and maintaing two different cases of broken tests was bit hard. Now that things work fine, let's add the flat manifest case. The test shows that things work fine in both cases. Differential Revision: https://phab.mercurial-scm.org/D4385