patch 8.0.0646: the hlsearch test fails on fast systems

Problem:    The hlsearch test fails on fast systems.
Solution:   Make the search pattern slower.  Fix that the old regexp engine
            doesn't timeout properly.

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    646,
 /**/
     645,
 /**/