Really bump version to 1.13.20~ (should not touch dpkg while sleep deprived).

configure.ac

@@ -1,7 +1,7 @@
 # Process this file with autoconf to produce a configure script.
 
 AC_PREREQ(2.59)
-AC_INIT([dpkg], [1.13.19], [debian-dpkg@lists.debian.org])
+AC_INIT([dpkg], [1.13.20~], [debian-dpkg@lists.debian.org])
 AC_CONFIG_SRCDIR([lib/dpkg.h])
 AC_CONFIG_MACRO_DIR([m4])
 AC_CONFIG_AUX_DIR([config])