patch 8.1.1251: no test for completion of mapping keys

Problem:    No test for completion of mapping keys.
Solution:   Add a test.  Also clean up the code.
diff --git a/src/version.c b/src/version.c
index 2fee19b..13b35d5 100644
--- a/src/version.c
+++ b/src/version.c
@@ -768,6 +768,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1251,
+/**/
     1250,
 /**/
     1249,