Stop use default command on complition

[y-marui]

If _{prog_name}_COMPLETE is set, the click will execute completion instead of normal flow.
On completion, adding the default command name to arg cause trouble.

[pneff]

Hi @sublee

I just came across this one in my attempt to implement auto-completion for simonw/llm (in simonw/llm#261). The proposed fix does work in that case as well.

Can I add anything to this pull request to get it merged? E.g. test coverage?

In the case of the LLM tool I observed the following. Without the fix, no completion is output:

$ env COMP_WORDS="llm" COMP_CWORD=1 _LLM_COMPLETE=bash_complete llm
(no_output)

Once I apply the fix, it works:

$ env COMP_WORDS="llm" COMP_CWORD=1 _LLM_COMPLETE=bash_complete llm
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