patch 8.1.1639: changing an autoload name into a file name is inefficient

Problem:    Changing an autoload name into a script file name is inefficient.
Solution:   Remember the last replaced #. (Ozaki Kiichi, closes #4618)
diff --git a/src/version.c b/src/version.c
index c835eee..25c6076 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1639,
+/**/
     1638,
 /**/
     1637,