patch 8.0.0020

Problem:    The regexp engines are not reentrant.
Solution:   Add regexec_T and save/restore the state when needed.

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    20,
 /**/
     19,
 /**/