Fix multiple command registration for non-bash shells

[evanunderscore]

Fixes a bug introduced in #288 where the first executable would be used to complete all passed executables if no script was specified.

[codecov-io]

Codecov Report

Merging #293 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
+ Coverage   82.63%   82.65%   +0.02%     
==========================================
  Files           7        7              
  Lines         737      738       +1     
==========================================
+ Hits          609      610       +1     
  Misses        128      128              

Impacted Files	Coverage Δ
argcomplete/shell_integration.py	100.00% <100.00%> (ø)

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