gitlab-backup: Use find in a way, that does not hide errors

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

contrib/gitlab-backup

@@ -56,10 +56,16 @@ backup_pull() {
     tar -C "$tmpd/gitlab_backup" -xf "$backup_tar"
     gzip -d "$tmpd/gitlab_backup/db/database.sql.gz"  # unzip so it is better stored in git
     # unpack tarballs so files are better stored in git
-    find "$tmpd/gitlab_backup" -maxdepth 1 -type f -name "*.tar.gz" -exec   \
-        sh -c 'mv {} {}.x && mkdir {} && tar xf {}.x -C {} && rm {}.x' ';'
-    find "$tmpd/gitlab_backup" -maxdepth 1 -type d -name "*.tar.gz" -empty -exec    \
-        touch {}/.gitlab-backup-keep ';'
+    find "$tmpd/gitlab_backup" -maxdepth 1 -type f -name "*.tar.gz" | \
+    while read tar; do
+        mv $tar $tar.x
+        mkdir $tar
+        tar xf $tar.x -C $tar
+        rm $tar.x
+
+        # keep empty dirs too
+        test -n "`ls -A $tar`" || touch $tar/.gitlab-backup-keep
+    done
 
 
     # 3. pull gitlab data into git-backup
@@ -88,10 +94,14 @@ backup_restore() {
     $GIT_BACKUP restore $HEAD gitlab/misc:"$tmpd/gitlab_backup"
     gzip "$tmpd/gitlab_backup/db/database.sql"  # gzip sql dump, as gitlab expects .gz
     # recreate tarballs from *.tar.gz directories
-    find "$tmpd/gitlab_backup" -maxdepth 1 -type d -name "*.tar.gz" -exec   \
-        rm -f {}/.gitlab-backup-keep ';'
-    find "$tmpd/gitlab_backup" -maxdepth 1 -type d -name "*.tar.gz" -exec   \
-        sh -c 'mv {} {}.x && tar cfz {} -C {}.x . && rm -rf {}.x' ';'
+    find "$tmpd/gitlab_backup" -maxdepth 1 -type d -name "*.tar.gz" | \
+    while read tar; do
+        rm -f $tar/.gitlab-backup-keep
+
+        mv $tar $tar.x
+        tar cfz $tar -C $tar.x .
+        rm -rf $tar.x
+    done
 
     # 2. find out backup timestamp as saved by gitlab
     backup_created_at=`grep :backup_created_at: "$tmpd/gitlab_backup/backup_information.yml" |