t5543: atomic push reports exit code failure

t5543: atomic push reports exit code failure

Add new test cases in t5543 to avoid ignoring the exit code of
git-receive-pack(1) during atomic push with "--porcelain" flag.

We'd typically notice this case because the refs would have their error
message set. But there is an edge case when pushing refs succeeds, but
git-receive-pack(1) exits with a non-zero exit code at a later point in
time due to another error. An atomic git-push(1) would ignore that error
code, and consequently it would return successfully and not print any
error message at all.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>