WIP: Introduce receive.fsck.skip-list

This config option can be used to specify the path to a file whose
lines list names of objects that should be ignored when fsck'ing
incoming objects.

This is extremely handy in case of legacy repositories where it would
cause more pain to change incorrect objects than to live with them
(e.g. a duplicate 'author' line in an early commit object).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fixup! WIP: Introduce receive.fsck.skip-list
TODO: safer type parsing

Maybe memcmp is better?

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>