Solve quoting issue with "su" (debian #683731) (upstream #4019)

A fix for debian #677410 on commit 29dcdbae introduces a new issue on wheezy
machines where some rsync_options gets out of the rsync command quotes when
invoked by su.

As reported, running backupninja through `bash -x` shows that the `--delete`
option from the rsync command is not included in the single quote for the `su
-c` command:

    nice -n 0 su -c '/usr/bin/rsync -av' --delete '--recursive
    --delete-excluded    //home/ /tmp/test//home//'

This patch attempt to solve that issue by using the intermediary variable
$command.

handlers/rsync.in

@@ -1136,9 +1136,10 @@ for SECTION in $include; do
   set_dest
 
   info "Syncing $SECTION on $dest_path..."
-  debug $nice $rsync ${rsync_options[@]} $filelist_flag $excludes $batch_option $orig $dest_path
+  command="$rsync ${rsync_options[@]} --delete-excluded $filelist_flag $excludes $batch_option $orig $dest_path"
+  debug $nice su -c "$command"
   set_pipefail
-  $nice su -c "$rsync ${rsync_options[@]} --delete-excluded $filelist_flag $excludes $batch_option $orig $dest_path" | tee -a $log
+  $nice su -c "$command" | tee -a $log
 
   check_rsync_exit_status $?
   restore_pipefail