patch 8.1.0980: extend() insufficiently tested

Problem:    extend() insufficiently tested.
Solution:   Add more tests. (Dominique Pelle, closes #4040)
diff --git a/src/version.c b/src/version.c
index a5a6f7a..e802aaf 100644
--- a/src/version.c
+++ b/src/version.c
@@ -780,6 +780,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    980,
+/**/
     979,
 /**/
     978,