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>
