It's adding a slight indirection by constructing a function when called,
but makes the completion functions more consistent, the signature easier
to read, and making the return type a [cobra.CompletionFunc] makes it
more transparent what it's intended for, and helps discovery of functions
that provide completion.
[cobra.CompletionFunc]: https://pkg.go.dev/github.com/spf13/cobra#CompletionFunc
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
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