Similar to the previous commit, enable object verification when fetching
objects. This covers usecases where Gitaly is fetching objects into
mirrors or updates object pools and stops corrupt or malign objects from
being accepted into repositories.

If mirroring a repository, then we'd now break if the remote repository
has corrupt objects, and similar for object pools and their primary
member. While this could be seen as a bug, git-fsck(1) nowadays also
includes checks to detect malign objects which are known to allow for
adverse behaviour. It is thus an important safety guard to protect
against an adversary sneaking in corrupt objects into mirrors via their
remote or into repository forks via the object pool. In case this is
causing problems, the route to go would thus be to ignore some of the
benign fsck checks like we already do for bad timezone checks.

Changelog: security