convert: Using --dest-type svn crashed, if the source repo used tags.

The convert extension requires puttags(self, tags) to return a sequence
for a multi-variable assignment.  If puttags implicitly returns None,
the code will break when trying to un-pack None for assignment.

hgext/convert/subversion.py

@@ -1158,3 +1158,4 b' class svn_sink(converter_sink, commandli'
 
     def puttags(self, tags):
         self.ui.warn(_('XXX TAGS NOT IMPLEMENTED YET\n'))
+        return None, None