fix: offer ancestors' persistent flags in the fish completion script - #2506

What type of PR is this?

• bug

What this PR does / why we need it:

In v3 a flag is persistent unless it is Local, so a subcommand accepts its ancestors' flags, and its help lists them under GLOBAL OPTIONS. ToFishCompletion only offered each command's own flags, and the root's flags only before any subcommand, so after a subcommand fish never suggested them:

cmd := &cli.Command{
	Name: "app",
	Flags: []cli.Flag{
		&cli.StringFlag{Name: "config", Aliases: []string{"c"}, TakesFile: true},
		&cli.BoolFlag{Name: "verbose"},
		&cli.BoolFlag{Name: "local-only", Local: true},
	},
	Commands: []*cli.Command{
		{Name: "deploy", Flags: []cli.Flag{&cli.BoolFlag{Name: "dry-run"}},
			Commands: []*cli.Command{{Name: "prod"}}},
	},
}

In fish 4.9.3 with the generated script, complete -C 'app deploy --' listed --dry-run --help, while app deploy --verbose prod runs and app deploy prod --help shows --config and --verbose under GLOBAL OPTIONS. The dynamic completion (app completion fish) already offers them, since DefaultCompleteWithFlags adds VisiblePersistentFlags.

• fish.go: prepareFishFlags now adds the command's VisiblePersistentFlags() that it doesn't define itself, the same way DefaultCompleteWithFlags does. VisiblePersistentFlags already leaves out Local flags and flags a subcommand redefines.
• testdata/expected-fish-full.fish and testdata/expected-help-output.txt: regenerated. Both only gain lines, the persistent flags under each subcommand, and no existing line changes.

With this change, complete -C gives the same flags with the static script as with the dynamic one at every subcommand level: app deploy -- and app deploy prod -- both list --config --dry-run --help --verbose, --local-only stays at the root, and app deploy -c completes file names.

Which issue(s) this PR fixes:

None, found while comparing the static and dynamic fish scripts.

Testing

• New TestFishCompletionPersistentFlags: a root flag is offered after a subcommand and a nested subcommand (fails on main), while a Local root flag and a root flag the subcommand redefines are not.
• Checked the generated script in fish 4.9.3, as above.
• make (generate, vet, tests, binary size, gfmrun), make lint and make v3diff pass. go test -race passes, as do the tests with urfave_cli_no_template, urfave_cli_no_docs and urfave_cli_no_suggest.

The change and the test were written with Claude Code (AI).

Release Notes

The fish completion script from ToFishCompletion now offers the persistent flags of a subcommand's ancestors, such as the root's flags.

🤖 Generated with Claude Code