You're better off asserting that your objects haven't changed (Which they weren't, and I agree that they should have been) and were valid in the first place (See above).
With snapshots, you'd invevitably want to dedupe them, which would be basically the same thing since it's append only, but with the dedupe infrastructure as another failure point.
The problem I think needs more attention here is ext4 silently corrupting data. ZFS has it exactly right with the built-in checksumming on write and read - it can't stop a disk going bad, but it can tell you exactly what's affected and _when_ - corruption would've been caught the moment the mirroring operation tried to read back bad data (and would've faulted the process, rather then happily return bad data).
Well, the KDE incident proves otherwise.
The next two paragraphs identified two things that they weren't doing that they should have been. Otherwise they'd just have lots of snapshots of bad data.
It would have been just as easy to push those corrupted repos to all of the backup tapes in the rotating snapshot set. A snapshotting filesystem could be a good backup (and seems to be what one of the sysadmins is pushing for).
But even more important is to fail fast and identify git repo corruption as soon as it can be detected so that further damage can be avoided.