Dirty workaround to display helpful error message on mistyped subcommands

When executing `gh pr re` (note the incomplete command name), Cobra
would just display the help text for `gh pr` on standard output, exit
with status 0, and not print any message that you have mistyped the
"re" subcommand. Each part of this behavior is wrong.

This workaround makes sure that the helpful error message is printed on
stderr:

    $ gh pr re
    unknown command "re" for "gh pr"

    Did you mean this?
            reopen
            ready
            review

However, the exit status is still 0, whereas it should be non-zero.
Since `HelpFunc` does not return an error argument, we cannot trigger an
error status from this workaround.

command/root.go

@@ -249,9 +249,33 @@ func determineBaseRepo(apiClient *api.Client, cmd *cobra.Command, ctx context.Co
 	return baseRepo, nil
 }
 
-func rootHelpFunc(command *cobra.Command, s []string) {
+func rootHelpFunc(command *cobra.Command, args []string) {
 	if command != RootCmd {
-		cobraDefaultHelpFunc(command, s)
+		// Display helpful error message in case subcommand name was mistyped.
+		// This matches Cobra's behavior for root command, which Cobra
+		// confusingly doesn't apply to nested commands.
+		if command.Parent() == RootCmd && len(args) >= 2 {
+			if command.SuggestionsMinimumDistance <= 0 {
+				command.SuggestionsMinimumDistance = 2
+			}
+			candidates := command.SuggestionsFor(args[1])
+
+			errOut := command.OutOrStderr()
+			fmt.Fprintf(errOut, "unknown command %q for %q\n", args[1], "gh "+args[0])
+
+			if len(candidates) > 0 {
+				fmt.Fprint(errOut, "\nDid you mean this?\n")
+				for _, c := range candidates {
+					fmt.Fprintf(errOut, "\t%s\n", c)
+				}
+				fmt.Fprint(errOut, "\n")
+			}
+
+			oldOut := command.OutOrStdout()
+			command.SetOut(errOut)
+			defer command.SetOut(oldOut)
+		}
+		cobraDefaultHelpFunc(command, args)
 		return
 	}