patch 8.1.0688: text properties are not restored by undo

Problem:    Text properties are not restored by undo.
Solution:   Also save text properties for undo.
diff --git a/src/version.c b/src/version.c
index e9ceeb5..3edb146 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    688,
+/**/
     687,
 /**/
     686,