This test expects "git push" to fail, thus it manually inverts that
local expected failure into a successful exit code for the test overall.
In doing so, it intentionally breaks the &&-chain. Modernize by
replacing manual exit code management with test_must_fail() and a normal
&&-chain.
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>