patch 8.1.0513: no error for set diffopt+=algorithm:

Problem:    No error for set diffopt+=algorithm:.
Solution:   Check for missing argument. (Hirohito Higashi, closes #3598)

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    513,
 /**/
     512,
 /**/