Make the script work with space-containing pathnames

while:

1. Showing the final tar command on stdout.

   I tried the approach to continue constructing a string
   containing the whole final tar command
   and include and escape paths with double quotes
   and store the whole tar command inside inside a variable.
   But this didn't work, tar terminated with a misterious error.
   After googling I found out the problem is "bash command inside variable".
   So now drop using a custom variable to store the final tar command inside.
   As recommended in http://mywiki.wooledge.org/BashFAQ/050
   and google:"bash command inside variable" .
   I chose the "set -x ... set +x" approach.
   As a sideeffect unwanted commands are shown as well (e.g. "set +x") .

   This is a reason to abandon bash as a scripting language
   and use Python or Perl or Ruby (or Korn ? or Ksh ?) instead in the future
   as someone noted on forums.

NOTE: TODO:
Drop using bash as a scripting language.
Rewrite this project in a different stronger scripting langugage.

NOTE: TODO:
Currently only one "--exclude=..." pattern can be used with this script.
Allow more such arguments to be passed to tar.

 # On branch bugfix--allow-spaces-in-pathnames
 # Changes to be committed:
 #	modified:   backup-script-tar-bz2.sh
 #
Merge branch 'bugfix--allow-spaces-in-pathnames'

Looks like the bugfix is tested enough and deployable.

 # Please enter a commit message to explain why this merge is necessary,
 # especially if it merges an updated upstream into a topic branch.
 #