All the tests asserting failure use 'test_must_fail', which simply
checks for a non-zero exit status, potentially hiding underlying bugs.
So, replace instances of 'test_must_fail' with 'test_expect_code' to
check the exit status explicitly, where appropriate.
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>