fix: make fish completions work

[yjp20]

What type of PR is this?

• bug

What this PR does / why we need it:

This PR fixes a bug where ./cli completion fish always outputs

# completion fish shell completion

function __fish_completion_no_subcommand --description 'Test if there has been any subcommand yet'
    for i in (commandline -opc)
        if contains -- $i
            return 1
        end
    end
    return 0
end

complete -c completion -n '__fish_completion_no_subcommand' -f -l help -s h -d 'show help'
complete -c completion -n '__fish_completion_no_subcommand' -f -l help -s h -d 'show help'

This is because the command used to generate the completions is always the one corresponding to the completion command, and it needs to know the root command.

Which issue(s) this PR fixes:

issue #2105

Testing

After making these changes, completions for my CLI generated successfully.

Release Notes

Fixes issue with fish completions which showed completions only for the `complete` command

[dearchap]

@yjp20 You dont need to make all these changes you could have done the following

https://github.com/urfave/cli/blob/main/command_setup.go#L92

completionCommand := buildCompletionCommand(cmd.Root().Name)

[yjp20]

You dont need to make all these changes you could have done the following
completionCommand := buildCompletionCommand(cmd.Root().Name)

I'm pretty sure this isn't sufficient, because you have to do c.ToFishCompletion() as a part of fish handler, so c here definitely has to be the rootCmd. You can check this out yourself if you'd like.

If you're averse to code changes, I could also change this such that the fish handler does c.Root().ToFishCompletion(), but frankly this feels a bit weirder to me, though I'm happy to make this change.

[dearchap]

Will check and get back to you

[dearchap]

Fixed by #2106