feat(help): add ShowSubcommandHelpAndExit

[knqyf263]

What type of PR is this?

• bug
• cleanup
• documentation
• feature

What this PR does / why we need it:

• Add ShowSubcommandHelpAndExit to help.go
• It is also useful like ShowCommandHelpAndExit and ShowAppHelpAndExit

Which issue(s) this PR fixes:

Fixes #1112

Special notes for your reviewer:

I didn't find any test for XXXHelpAndExit.

Testing

I prepared a sample program.

Release Notes

Added ShowSubcommandHelpAndExit to print help for the given subcommand and exit with exit code.

[rliebz]

I'm not a huge fan of increasing the surface area of the public API here to turn a two-liner into a one-liner.

Is there a reason why doing these calls manually is problematic?

[knqyf263]

This is because urfave/cli already has the following public APIs. I think they are useful even though it turns a two-liner into a one-liner.

cli/help.go

Lines 67 to 71 in 74af85a

	// ShowAppHelpAndExit - Prints the list of subcommands for the app and exits with exit code.
	func ShowAppHelpAndExit(c *Context, exitCode int) {
	_ = ShowAppHelp(c)
	os.Exit(exitCode)
	}

cli/help.go

Lines 182 to 186 in 74af85a

	// ShowCommandHelpAndExit - exits with code after showing help
	func ShowCommandHelpAndExit(c *Context, command string, code int) {
	_ = ShowCommandHelp(c, command)
	os.Exit(code)
	}

If ShowSubcommandHelpAndExit doesn't make sense, I feel like the above APIs also don't make sense, just reducing one line. I thought they are the same, but is there any difference between ShowSubcommandHelp and ShowCommandHelp?

[rliebz]

Sorry for the delay here. You're right—having 2/3 of these functions is an obvious omission.

Thanks for the contribution!

[asahasrabuddhe]

LGTM