patch 8.1.0669: the ex_sign() function is too long

Problem:    The ex_sign() function is too long.
Solution:   Refactor the function.  Add a bit more testing. (Yegappan
            Lakshmanan, closes #3745)
diff --git a/src/version.c b/src/version.c
index 486db73..fe98bc4 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    669,
+/**/
     668,
 /**/
     667,