dpkg-gensymbols: improve diff label again.

Use "$source_file ($package $arch)".

Signed-off-by: Modestas Vainius <modestas@vainius.eu>

debian/changelog

@@ -34,6 +34,9 @@ dpkg (1.15.4) UNRELEASED; urgency=low
     sub-directories. Closes: #535138
   * Upgrade Standards-Version to 3.8.2 (no changes).
 
+  [ Modestas Vainius ]
+  * Provide a meaningful label for dpkg-gensymbols diff.
+
   [ Updated dpkg translations ]
   * Asturian (Marcos Alvarez Costales). Closes: #535327
   * French (Christian Perrier).

scripts/dpkg-gensymbols.pl

@@ -276,7 +276,10 @@ if ($compare) {
 			$output);
 	    }
 	    my ($a, $b) = ($before->filename, $after->filename);
-	    system("diff", "-u", "-L", "dpkg-gensymbols_${oppackage}_${host_arch}", $a, $b) if -x "/usr/bin/diff";
+	    my $diff_label = sprintf("%s (%s %s)",
+	    ($ref_symfile->{file}) ? $ref_symfile->{file} : "new_symbol_file",
+	    $oppackage, $host_arch);
+	    system("diff", "-u", "-L", $diff_label, $a, $b) if -x "/usr/bin/diff";
 	}
     }
 }