Fix an issue when both export-dir and tarball-dir is set

When the export-dir is set, the git tree is archived and then re-extracted
into the new place.  Unfortunately, different directory is in use to create
the archive (output_dir) and then to extract it (tarball_dir, controlled by
the tarball-dir option).  This means that if no orig.tar.gz is provided in
the tarball_dir directory, the extraction will fail.

gbp/scripts/buildpackage.py

@@ -538,7 +538,7 @@ def main(argv):
 
             # Export to another build dir if requested:
             if options.export_dir:
-                export_source(repo, tree, source, options, tmp_dir, tarball_dir)
+                export_source(repo, tree, source, options, tmp_dir, output_dir)
 
                 # Run postexport hook
                 if options.postexport: