fsck: document that skipList input must be unabbreviated

fsck: document that skipList input must be unabbreviated

Abbreviating the SHA-1s in the skipList input has never worked, but
the documentation hasn't unambiguously stated that this is an error,
and there was no test for it.

Let's fix both since it would be easy for some later refactoring
e.g. switch to accidentally switch to a looser OID parsing function,
causing the tests before this change to pass, but for older versions
of git to be incompatible with the new skipList format.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>