Add one optional positional parameter

Changes:

1. Add test if number of parameters is correct.
   Create function test_if_number_of_parameters_is_correct() .
   Self-descriptive name.

2. Add one optional positional parameter.
   Pass the parameter to the tar's CLI optional parameter "--exclude="... .

3. Other minor enhancements.

 # On branch record-original-older-untracked-files
 # Your branch is up-to-date with 'origin/record-original-older-untracked-files'.
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 # Changes to be committed:
 #	modified:   backup-script-tar-bz2.sh
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