operations: fix invalid UTF-8 when truncating file names when not using --inplace

Before this change, when not using --inplace, rclone could generate
invalid file names when truncating file names to fit within the
character size limits.

This fixes it by taking care to truncate on UTF-8 character
boundaries.

See: https://forum.rclone.org/t/ssh-fx-failure-when-copying-file-with-nonstandard-characters-to-sftp-remote-with-ntfs-drive/42560/

fs/operations/copy_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"crypto/rand"
 	"errors"
+	"fmt"
 	"strings"
 	"testing"
 
@@ -15,6 +16,85 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestTruncateString(t *testing.T) {
+	for _, test := range []struct {
+		in   string
+		n    int
+		want string
+	}{
+		{
+			in:   "",
+			n:    0,
+			want: "",
+		}, {
+			in:   "Hello World",
+			n:    5,
+			want: "Hello",
+		}, {
+			in:   "ááááá",
+			n:    5,
+			want: "áá",
+		}, {
+			in:   "ááááá\xFF\xFF",
+			n:    5,
+			want: "áá\xc3",
+		}, {
+			in:   "世世世世世",
+			n:    7,
+			want: "世世",
+		}, {
+			in:   "🙂🙂🙂🙂🙂",
+			n:    16,
+			want: "🙂🙂🙂🙂",
+		}, {
+			in:   "🙂🙂🙂🙂🙂",
+			n:    15,
+			want: "🙂🙂🙂",
+		}, {
+			in:   "🙂🙂🙂🙂🙂",
+			n:    14,
+			want: "🙂🙂🙂",
+		}, {
+			in:   "🙂🙂🙂🙂🙂",
+			n:    13,
+			want: "🙂🙂🙂",
+		}, {
+			in:   "🙂🙂🙂🙂🙂",
+			n:    12,
+			want: "🙂🙂🙂",
+		}, {
+			in:   "🙂🙂🙂🙂🙂",
+			n:    11,
+			want: "🙂🙂",
+		}, {
+			in:   "𝓝𝓸𝓫𝓸𝓭𝔂 𝓲𝓼 𝓱𝓸𝓶𝓮 ᴬ ⱽⁱˢⁱᵗ ᶠʳᵒᵐ ᵗʰᵉ ⱽⁱˢⁱᵒⁿᵃʳʸ",
+			n:    100,
+			want: "𝓝𝓸𝓫𝓸𝓭𝔂 𝓲𝓼 𝓱𝓸𝓶𝓮 ᴬ ⱽⁱˢⁱᵗ ᶠʳᵒᵐ ᵗʰᵉ ⱽⁱˢ",
+		}, {
+			in:   "a𝓝𝓸𝓫𝓸𝓭𝔂 𝓲𝓼 𝓱𝓸𝓶𝓮 ᴬ ⱽⁱˢⁱᵗ ᶠʳᵒᵐ ᵗʰᵉ ⱽⁱˢⁱᵒⁿᵃʳʸ",
+			n:    100,
+			want: "a𝓝𝓸𝓫𝓸𝓭𝔂 𝓲𝓼 𝓱𝓸𝓶𝓮 ᴬ ⱽⁱˢⁱᵗ ᶠʳᵒᵐ ᵗʰᵉ ⱽⁱˢ",
+		}, {
+			in:   "aa𝓝𝓸𝓫𝓸𝓭𝔂 𝓲𝓼 𝓱𝓸𝓶𝓮 ᴬ ⱽⁱˢⁱᵗ ᶠʳᵒᵐ ᵗʰᵉ ⱽⁱˢⁱᵒⁿᵃʳʸ",
+			n:    100,
+			want: "aa𝓝𝓸𝓫𝓸𝓭𝔂 𝓲𝓼 𝓱𝓸𝓶𝓮 ᴬ ⱽⁱˢⁱᵗ ᶠʳᵒᵐ ᵗʰᵉ ⱽⁱ",
+		}, {
+			in:   "aaa𝓝𝓸𝓫𝓸𝓭𝔂 𝓲𝓼 𝓱𝓸𝓶𝓮 ᴬ ⱽⁱˢⁱᵗ ᶠʳᵒᵐ ᵗʰᵉ ⱽⁱˢⁱᵒⁿᵃʳʸ",
+			n:    100,
+			want: "aaa𝓝𝓸𝓫𝓸𝓭𝔂 𝓲𝓼 𝓱𝓸𝓶𝓮 ᴬ ⱽⁱˢⁱᵗ ᶠʳᵒᵐ ᵗʰᵉ ⱽⁱ",
+		}, {
+			in:   "aaaa𝓝𝓸𝓫𝓸𝓭𝔂 𝓲𝓼 𝓱𝓸𝓶𝓮 ᴬ ⱽⁱˢⁱᵗ ᶠʳᵒᵐ ᵗʰᵉ ⱽⁱˢⁱᵒⁿᵃʳʸ",
+			n:    100,
+			want: "aaaa𝓝𝓸𝓫𝓸𝓭𝔂 𝓲𝓼 𝓱𝓸𝓶𝓮 ᴬ ⱽⁱˢⁱᵗ ᶠʳᵒᵐ ᵗʰᵉ ⱽ",
+		},
+	} {
+		got := operations.TruncateString(test.in, test.n)
+		assert.Equal(t, test.want, got, fmt.Sprintf("In %q", test.in))
+		assert.LessOrEqual(t, len(got), test.n)
+		assert.GreaterOrEqual(t, len(got), test.n-3)
+	}
+}
+
 func TestCopyFile(t *testing.T) {
 	ctx := context.Background()
 	r := fstest.NewRun(t)