[Aubaid12]

Follow-up to the suggestion on #608. Now that --output only accepts json, human and custom, the shell can complete those values instead of suggesting file names.

It's one RegisterFlagCompletionFunc call on the root command using cobra.FixedCompletions with common.OutputFormats, so the list stays in one place. Since --output is a persistent flag, every subcommand gets it too.

$ civo __complete instance ls --output ""
json
human
custom
:4
Completion ended with directive: ShellCompDirectiveNoFileComp

I also tried the generated bash script, and civo kubernetes ls -o c<TAB> completes to custom. The other shells go through the same __complete command, so they pick it up as well.

Tests: TestOutputFlagCompletesFormats in cmd/root_test.go checks the completion on the root command and on a subcommand, and fails if the registration is removed. go test ./... and golangci-lint pass.

[fulviodenza]

Thanks for the PRs opened, I'm going to release these in the next tag