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)

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1639,
 /**/
     1638,
 /**/