push tests: fix logic error in "push" test assertion

Fix a logic error that's been here since this test was added in dbfeddb12e ("push: require force for refs under refs/tags/", 2012-11-29). The intent of this test is to force-create a new tag pointing to HEAD~, and then assert that pushing it doesn't work without --force. Instead, the code was not creating a new tag at all, and then failing to push the previous tag for the unrelated reason of providing a refspec that doesn't make any sense. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>