patch 8.2.0844: text properties crossing lines not handled correctly

Problem:    Text properties crossing lines not handled correctly.
Solution:   When saving for undo include an extra line when needed and do not
            adjust properties when undoing. (Axel Forsman, closes #5875)

src/structs.h

@@ -742,6 +742,15 @@ typedef struct memline
 #endif
 } memline_T;
 
+// Values for the flags argument of ml_delete_flags().
+#define ML_DEL_MESSAGE	    1	// may give a "No lines in buffer" message
+#define ML_DEL_UNDO	    2	// called from undo, do not update textprops
+
+// Values for the flags argument of ml_append_int().
+#define ML_APPEND_NEW	    1	// starting to edit a new file
+#define ML_APPEND_MARK	    2	// mark the new line
+#define ML_APPEND_UNDO	    4	// called from undo
+
 
 /*
  * Structure defining text properties.  These stick with the text.