When fsck.<msg-id> is set to an unknown value it'll cause "fsck" to
die, but the same is not true of the "fetch" and "receive"
variants. Document this and test for it.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>