convert: add missing import of util, needed for svn < 1.6

hgext/convert/transport.py

@@ -18,6 +18,7 b''
 # along with this program; if not, write to the Free Software
 # Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 
+from mercurial import util
 from svn.core import SubversionException, Pool
 import svn.ra
 import svn.client