dpkg-source: add Bug-Ubuntu field in DEP-3 template

The DEP-3 template is provided in the automatic header of patches in 3.0
(quilt) source packages.

debian/changelog

@@ -12,6 +12,9 @@ dpkg (1.15.7.2) UNRELEASED; urgency=low
     it's kept.
   * Let dpkg-buildflags error out when a required parameter is missing.
     Closes: #579722
+  * Add Bug-Ubuntu field in DEP-3 template provided in the automatic header
+    of patches in 3.0 (quilt) source packages. Thanks to Benjamin Drung
+    <bdrung@ubuntu.com> for the patch. Closes: #578002
 
   [ Guillem Jover ]
   * Add powerpcspe support to ostable and triplettable.

scripts/Dpkg/Source/Package/V3/quilt.pm

@@ -378,6 +378,7 @@ are templates for supplementary fields that you might want to add:
 Origin: <vendor|upstream|other>, <url of original patch>
 Bug: <url in upstream bugtracker>
 Bug-Debian: http://bugs.debian.org/<bugnumber>
+Bug-Ubuntu: https://launchpad.net/bugs/<bugnumber>
 Forwarded: <no|not-needed|url proving that it has been forwarded>
 Reviewed-By: <name and email of someone who approved the patch>
 Last-Update: <YYYY-MM-DD>\n\n";