dpkg-source: correctly create .pc/.quilt_series with alternate series files

If you have debian/patches/ubuntu.series and you were unpacking the source
package on ubuntu, quilt was still directed to debian/patches/series
instead of debian/patches/ubuntu.series.

debian/changelog

@@ -7,6 +7,9 @@ dpkg (1.15.8.6) UNRELEASED; urgency=low
   * Fix Dpkg::Version's handling of version with a debian revision but an
     empty version (e.g. "-0.1"). Thanks to James Vega <jamessan@debian.org>
     for the patch. Closes: #597651
+  * With "3.0 (quilt)" source package, create .pc/.quilt_series with the
+    correct series file if the source package provides vendor specific patch
+    sets.
 
   [ Updated man page translations ]
   * French (Christian Perrier).

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

@@ -154,7 +154,9 @@ sub create_quilt_db {
     $file = File::Spec->catfile($db_dir, ".quilt_series");
     if (not -e $file) {
         open(QSERIES, ">", $file) or syserr(_g("cannot write %s"), $file);
-        print QSERIES "series\n";
+        my $series = $self->get_series_file($dir) || "series";
+        $series = (File::Spec->splitpath($series))[2];
+        print QSERIES "$series\n";
         close(QSERIES);
     }
 }