Use usage template in subcommand help

[meatballhat]

What type of PR is this?

• bug

What this PR does / why we need it:

Use existing usage template in subcommand help to remove (confusing) difference between templates.

Which issue(s) this PR fixes:

Closes #1879

[BlackHole1]

LGTM

[vimiix]

@meatballhat Do we need to display different formats depending on whether there are subcommands?

For example, fix the templ variable in ShowSubcommandHelp function like this:

templ := SubcommandHelpTemplate
if cCtx.Command != nil {
  if cCtx.Command.CustomHelpTemplate != "" {
    templ = cCtx.Command.CustomHelpTemplate
  } else if len(cCtx.Command.VisibleCommands()) == 0 {
    templ = CommandHelpTemplate
  }
}

[meatballhat]

@vimiix good point! I'd like to see if that logic can be part of "usageTemplate" 👀