fsck: test & document {fetch,receive}.fsck.* config fallback

fsck: test & document {fetch,receive}.fsck.* config fallback

Test and document that the {fetch,receive}.fsck.* family of variables
doesn't fall back on the corresponding .fsck.* variables.

This was alluded to in the existing documentation by saying that
"receive" looks at receive.fsck.* and "fsck" looks at fsck.* etc., but
it wasn't explicitly stated that there was no fallback, and if you'd
e.g. like to configure the skipList you need to do that for all three.

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