push tests: use spaces in interpolated string

The quoted -m'msg' option would mean the same as -mmsg when passed through the test_force_push_tag helper. Let's instead use a string with spaces in it, to have a working example in case we need to pass other whitespace-delimited arguments to git-tag. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>