fs: Add global flag '--color' to control terminal colors

* fs: add TerminalColorMode type
* fs: add new config(flags) for TerminalColorMode
* lib/terminal: use TerminalColorMode to determine how to handle colors
* Add documentation for '--terminal-color-mode'
* tree: remove obsolete --color replaced by global --color

This changes the default behaviour of tree. It now displays colors by
default instead of only displaying them when the flag -C/--color was
active. Old behaviour (no color) can be achieved by setting --color to
'never'.

Fixes: #6604

fs/terminalcolormode_test.go

@@ -0,0 +1,74 @@
+package fs
+
+import (
+	"encoding/json"
+	"strconv"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestTerminalColorModeString(t *testing.T) {
+	for _, test := range []struct {
+		in   TerminalColorMode
+		want string
+	}{
+		{TerminalColorModeAuto, "AUTO"},
+		{TerminalColorModeAlways, "ALWAYS"},
+		{TerminalColorModeNever, "NEVER"},
+		{36, "TerminalColorMode(36)"},
+	} {
+		tcm := test.in
+		assert.Equal(t, test.want, tcm.String(), test.in)
+	}
+}
+
+func TestTerminalColorModeSet(t *testing.T) {
+	for _, test := range []struct {
+		in          string
+		want        TerminalColorMode
+		expectError bool
+	}{
+		{"auto", TerminalColorModeAuto, false},
+		{"ALWAYS", TerminalColorModeAlways, false},
+		{"Never", TerminalColorModeNever, false},
+		{"INVALID", 0, true},
+	} {
+		tcm := TerminalColorMode(0)
+		err := tcm.Set(test.in)
+		if test.expectError {
+			require.Error(t, err, test.in)
+		} else {
+			require.NoError(t, err, test.in)
+		}
+		assert.Equal(t, test.want, tcm, test.in)
+	}
+}
+
+func TestTerminalColorModeUnmarshalJSON(t *testing.T) {
+	for _, test := range []struct {
+		in          string
+		want        TerminalColorMode
+		expectError bool
+	}{
+		{`"auto"`, TerminalColorModeAuto, false},
+		{`"ALWAYS"`, TerminalColorModeAlways, false},
+		{`"Never"`, TerminalColorModeNever, false},
+		{`"Invalid"`, 0, true},
+		{strconv.Itoa(int(TerminalColorModeAuto)), TerminalColorModeAuto, false},
+		{strconv.Itoa(int(TerminalColorModeAlways)), TerminalColorModeAlways, false},
+		{strconv.Itoa(int(TerminalColorModeNever)), TerminalColorModeNever, false},
+		{`99`, 0, true},
+		{`-99`, 0, true},
+	} {
+		var tcm TerminalColorMode
+		err := json.Unmarshal([]byte(test.in), &tcm)
+		if test.expectError {
+			require.Error(t, err, test.in)
+		} else {
+			require.NoError(t, err, test.in)
+		}
+		assert.Equal(t, test.want, tcm, test.in)
+	}
+}