In 495bd2fa (gitlab-backup: Unpack *.tar.gz before storing them in git)
we used find(1) to find *.tar.gz and unpack/repack them on
backup/restore. However `find -exec ...` does not stop on errors and
does not report them. Compare:

    ---- 8< ---- x.sh
    #!/bin/bash -e

    echo AAA
    find . -exec false ';'
    echo BBB
    ---- 8< ----

    ---- 8< ---- y.sh
    #!/bin/bash -e

    echo XXX
    find . | \
    while read F; do
        false
    done
    echo YYY
    ---- 8< ----

    $ ./x.sh
    AAA
    BBB
    $ echo $?
    0

    $ ./y.sh
    XXX
    $ echo $?
    1

So we switch to second style where find passes entries to processing
program via channel. This second new style is also more clean, in my view,
because listing and processing parts are now more better structured.

/cc @kazuhiko