OK, it seems that I managed to make this test pass under poison build (see https://travis-ci.org/git/git/jobs/315658242)
Please check https://github.com/git/git/commit/e5c5e24ad91a75b5a70c056fe6c6e3bfb55b56fc and sprinkle its fix to whichever original commits in the series that need fixing. Thanks.
