Starting from 8.2 GitLab backups uploads and other directories not just
as set of files, but as one tarball:

    https://gitlab.com/gitlab-org/gitlab-ce/commit/d3734fbd

and this does not play well with git - now objects are stored as a
one big whole, compressed, so git cannot find good deltas.

So to help git properly deltify and find duplicates, let's unpack/repack
the archives, the same way we already do for database.sql.gz