cvs2svn errors when branch is rooted in a 'dead' revision

In

   http://www.contactor.se/~dast/svn/archive-2003-07/0886.shtml

Mike Wilcox reports a backtrace ending like this, both with a
(single-component) --trunk option and without:

       File "cvs2svn.py", line 1481, in copy_descend
         dumper.copy_path(src_path, parent_rev, copy_dst, val)
       File "cvs2svn.py", line 932, in copy_path
         entries)
       File "cvs2svn.py", line 632, in change_path
         for ent in new_val.keys():
   AttributeError: 'NoneType' object has no attribute 'keys'

He also said:

   Tentatively, this crash would seem to be caused by the recent
   'tag' changes, as this part of the repository has been
   converted successfully before, when 'only' the 'branch' code
   had been created (sorry, using the word 'only' to describe
   the branch-handling code seems kinda lame :-) ).  I'd say it
   worked in revision 6372, but doesn't in 6455.

   I can provide, privately, the files that cause the problem;
   I can narrow down the behavioural change to a better revision
   than between 6372 and 6455; I can create a new issue, or tag
   some details onto #1409. Let me know if any of these are useful
   :-)

I asked for the reproduction data, and he sent it to me, sorry can't
post it here of course.  I'll try to construct an untainted regression
test based on it.