patch 8.2.3689: ex_let_one() is too long

Problem:    ex_let_one() is too long.
Solution:   Split into multiple functions.
diff --git a/src/version.c b/src/version.c
index f43b33c..deefcf0 100644
--- a/src/version.c
+++ b/src/version.c
@@ -758,6 +758,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3689,
+/**/
     3688,
 /**/
     3687,