feat(fish): implement ShellCompDirectiveFilterDirs

[thaJeztah]

Fish supports directory-only completions through __fish_complete_directories, but Cobra's generated completion script treated this directive as unsupported.

Use Fish's native directory completion when ShellCompDirectiveFilterDirs is set.

[thaJeztah]

Failure is unrelated to my changes, but perhaps GolangCi-lint was updated;

  Error: completions.go:958:24: G703: Path traversal via taint analysis (gosec)
  		f, err := os.OpenFile(path,
  		                     ^

[ccoVeille]

I'm unsure why this line was removed

[thaJeztah]

I need to double check when I'm back at my computer, but I think the flow was; these options set a "bash" annotation on the flag, but the code further down converts that to setting autocompletion for the flag, using the ShellCompDirectiveFilterDirs directive.

So they were ignored because that directive wasn't functional 😅

[thaJeztah]

Right, so MarkFlagDirname sets the BashCompSubdirsInDir annotation;

cobra/shell_completions.go

Lines 96 to 98 in 61968e8

	func MarkFlagDirname(flags *pflag.FlagSet, name string) error {
	return flags.SetAnnotation(name, BashCompSubdirsInDir, []string{})
	}

Which then gets translated to ShellCompDirectiveFilterDirs here;

cobra/completions.go

Lines 416 to 423 in 61968e8

	if subDir, present := flag.Annotations[BashCompSubdirsInDir]; present {
	if len(subDir) == 1 {
	// Directory completion from within a directory
	return finalCmd, subDir, ShellCompDirectiveFilterDirs, nil
	}
	// Directory completion
	return finalCmd, []Completion{}, ShellCompDirectiveFilterDirs, nil
	}

The ShellCompDirectiveFilterDirs previously didn't take effect on Fish, but with this change does