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)
diff --git a/src/version.c b/src/version.c
index 3639702..3ed258f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    513,
+/**/
     512,
 /**/
     511,