use errors.Is() to check for errors (#1730)

Since go 1.13 you can wrap errors. This make it no longer possible to
compare with `==`, instead you have to compare with `errors.Is()`.

I noticed this problem because -h was no longer working after I stared
wrapping the errors in my custom FlagErrorFunc function.
Note that this is only a problem when a custom help flag is defined.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

command.go

@@ -18,6 +18,7 @@ package cobra
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -990,7 +991,7 @@ func (c *Command) ExecuteC() (cmd *Command, err error) {
 	if err != nil {
 		// Always show help if requested, even if SilenceErrors is in
 		// effect
-		if err == flag.ErrHelp {
+		if errors.Is(err, flag.ErrHelp) {
 			cmd.HelpFunc()(cmd, args)
 			return cmd, nil
 		}

command_test.go

@@ -1723,6 +1723,38 @@ func TestFlagErrorFunc(t *testing.T) {
 	}
 }
 
+func TestFlagErrorFuncHelp(t *testing.T) {
+	c := &Command{Use: "c", Run: emptyRun}
+	c.PersistentFlags().Bool("help", false, "help for c")
+	c.SetFlagErrorFunc(func(_ *Command, err error) error {
+		return fmt.Errorf("wrap error: %w", err)
+	})
+
+	out, err := executeCommand(c, "--help")
+	if err != nil {
+		t.Errorf("--help should not fail: %v", err)
+	}
+
+	expected := `Usage:
+  c [flags]
+
+Flags:
+      --help   help for c
+`
+	if out != expected {
+		t.Errorf("Expected: %v, got: %v", expected, out)
+	}
+
+	out, err = executeCommand(c, "-h")
+	if err != nil {
+		t.Errorf("-h should not fail: %v", err)
+	}
+
+	if out != expected {
+		t.Errorf("Expected: %v, got: %v", expected, out)
+	}
+}
+
 // TestSortedFlags checks,
 // if cmd.LocalFlags() is unsorted when cmd.Flags().SortFlags set to false.
 // Related to https://github.com/spf13/cobra/issues/404.