patch 8.1.0519: cannot save and restore the tag stack

Problem:    Cannot save and restore the tag stack.
Solution:   Add gettagstack() and settagstack(). (Yegappan Lakshmanan,
            closes #3604)

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    519,
 /**/
     518,
 /**/