Use command passed into p.Parse(...) write methods

It is currently impossible to programatically write help and usage
messages for subcommands, due to parser.WriteHelp and parser.WriteUsage
not taking the state of the parser into account.

Check for the existence of p.lastCmd and use it for the writers when
available.

Enables ability to write unit tests for subcommand help.

usage.go

@@ -27,7 +27,11 @@ func (p *Parser) failWithCommand(msg string, cmd *command) {
 
 // WriteUsage writes usage information to the given writer
 func (p *Parser) WriteUsage(w io.Writer) {
-	p.writeUsageForCommand(w, p.cmd)
+	cmd := p.cmd
+	if p.lastCmd != nil {
+		cmd = p.lastCmd
+	}
+	p.writeUsageForCommand(w, cmd)
 }
 
 // writeUsageForCommand writes usage information for the given subcommand
@@ -116,7 +120,11 @@ func printTwoCols(w io.Writer, left, help string, defaultVal string) {
 
 // WriteHelp writes the usage string followed by the full help string for each option
 func (p *Parser) WriteHelp(w io.Writer) {
-	p.writeHelpForCommand(w, p.cmd)
+	cmd := p.cmd
+	if p.lastCmd != nil {
+		cmd = p.lastCmd
+	}
+	p.writeHelpForCommand(w, cmd)
 }
 
 // writeHelp writes the usage string for the given subcommand

usage_test.go

@@ -266,8 +266,45 @@ Options:
 	p, err := NewParser(Config{}, &args)
 	require.NoError(t, err)
 
-	os.Args[0] = "example"
 	var help bytes.Buffer
 	p.WriteHelp(&help)
 	assert.Equal(t, expectedHelp, help.String())
 }
+
+func TestUsagWithNestedSubcommands(t *testing.T) {
+	expectedHelp := `Usage: example child nested [--enable] OUTPUT
+
+Positional arguments:
+  OUTPUT
+
+Options:
+  --enable
+
+Global options:
+  --values VALUES        Values
+  --verbose, -v          verbosity level
+  --help, -h             display this help and exit
+`
+
+	var args struct {
+		Verbose bool `arg:"-v" help:"verbosity level"`
+		Child   *struct {
+			Values []float64 `help:"Values"`
+			Nested *struct {
+				Enable bool
+				Output string `arg:"positional,required"`
+			} `arg:"subcommand:nested"`
+		} `arg:"subcommand:child"`
+	}
+
+	os.Args[0] = "example"
+	p, err := NewParser(Config{}, &args)
+	require.NoError(t, err)
+
+	err = p.Parse([]string{"child", "nested", "value"})
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	fmt.Println(help.String())
+	assert.Equal(t, expectedHelp, help.String())
+}