fsck: store repository in fsck options

fsck: store repository in fsck options

The fsck subsystem relies on `the_repository` quite a bit. While we
could of course explicitly pass a repository down the callchain, we
already have a `struct fsck_options` that we pass to almost all
functions.

Extend the options to also store the repository to make it readily
available.

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