Set default in parameter expasion to avoid do-rebase exit due an unset var
    
    The script has set -u so calling do-rebase --help fails with the following:
    
      $ ./do-rebase --help
      ./do-rebase: line 16: $1: unbound variable
    
    Avoid this by setting a default value when doing the parameter expansion
    for the positional parameter $1 passed to the usage() function.
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>