Since objects are shared between backed up repositories, it is important
to make sure we do not pull a broken object once, thus programming
future corruption of that object after restore in all repositories which
use it.

Object corruption could happen for two reasons:

    - plain storage corruption, or
    - someone intentionally pushing corrupted object with known sha1 to
      any repository.

Second case is even more dangerous, as it potentially allows attacker to
change data in not-available-to-him repositories.

Now objects are checked on pull, and if corrupt, git-backup complains,
e.g. this way:

    RuntimeError: git -c fetch.fsckObjects=true fetch --no-tags ../D/corrupt.git refs/*:refs/backup/20151014-1914/aaa/corrupt.git/*
    error: inflate: data stream error (incorrect data check)
    fatal: loose object 52baccfe8479b61c2a0d5447bc0a6bf7c6827c60 (stored in ./objects/52/baccfe8479b61c2a0d5447bc0a6bf7c6827c60) is corrupt
    fatal: The remote end hung up unexpectedly