dpkg-genchanges: skip warning about lower version for backports

Backports have by definition a version number lower than the original
version (to enable upgrades from backports to official packages). So skip
the corresponding warning when backports are detected thanks to the
usual version suffixes ~bpo (backports.org) and ~vola (volatile.d.o).

debian/changelog

@@ -36,6 +36,8 @@ dpkg (1.15.3) UNRELEASED; urgency=low
     Closes: #530769
   * In dpkg-vendor, allow to use dashes instead of spaces in vendor
     filenames. Closes: #532222
+  * Skip dpkg-genchanges' warning about lower version numbers for backports
+    (recognized by ~bpo or ~vola in their version number). Closes: #525115
 
   [ Joachim Breitner ]
   * Warn about unused substvars in dpkg-gencontrol. Closes: #532760

scripts/dpkg-genchanges.pl

@@ -211,7 +211,9 @@ $substvars->parse($varlistfile) if -e $varlistfile;
 if (defined($prev_changelog) and
     compare_versions($changelog->{"Version"}, '<<', $prev_changelog->{"Version"})) {
     warning(_g("the current version (%s) is smaller than the previous one (%s)"),
-	$changelog->{"Version"}, $prev_changelog->{"Version"});
+	$changelog->{"Version"}, $prev_changelog->{"Version"})
+        # ~bpo and ~vola are backports and have lower version number by definition
+        unless $changelog->{"Version"} =~ /~(?:bpo|vola)/;
 }
 
 if (not is_sourceonly) {