restore: support sparse restores also on windows

internal/restorer/truncate_windows.go

@@ -0,0 +1,19 @@
+package restorer
+
+import (
+	"os"
+
+	"github.com/restic/restic/internal/debug"
+	"golang.org/x/sys/windows"
+)
+
+func truncateSparse(f *os.File, size int64) error {
+	// try setting the sparse file attribute, but ignore the error if it fails
+	var t uint32
+	err := windows.DeviceIoControl(windows.Handle(f.Fd()), windows.FSCTL_SET_SPARSE, nil, 0, nil, 0, &t, nil)
+	if err != nil {
+		debug.Log("failed to set sparse attribute for %v: %v", f.Name(), err)
+	}
+
+	return f.Truncate(size)
+}