Tips for debugging fast-import front-ends

I was wondering if anyone had some handy tips for debugging fast-import based importers?

I'm trying to import an SVN repository that was initially created by cvs2svn and, since we started using branches quite heavily _before_ switching to SVN, there is some quite messy history.

At some point in the "messy" phase I get an error from fast-import saying:

fatal: path foo not in branch

This seems to happen when I attempt to copy a file that is also changed in the same commit. What I would like to do is find out what things look like just before that point - the problem is that fast-import seems to not create anything on failure. So, apart restarting the import and stopping just before the broken commit, is there any way to find out what fast-import thinks the branch looks like?

Also, does anyone have any handy hints 'n' tips for working with fast-import?