patch 8.1.0644: finding next sign ID is inefficient

Problem:    Finding next sign ID is inefficient.
Solution:   Add next_sign_id. (Yegappan Lakshmanan, closes #3717)
diff --git a/src/version.c b/src/version.c
index 06f7a8a..cec9bc6 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    644,
+/**/
     643,
 /**/
     642,