Fix short option argument handling in autocompletion scripts

[dwalluck]

Closes #470.

[codecov-io]

Codecov Report

Merging #471 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #471      +/-   ##
============================================
- Coverage     89.12%   89.11%   -0.02%     
  Complexity      280      280              
============================================
  Files             4        4              
  Lines          3835     3831       -4     
  Branches        929      929              
============================================
- Hits           3418     3414       -4     
  Misses          207      207              
  Partials        210      210

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/picocli/AutoComplete.java	86.55% <100%> (-0.18%)	80 <0> (ø)

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