Rails3: scm: cvs: fix error of test_fetch_changesets_incremental at unit model test

On Rails 3.1, ActiveRecord::HasManyThroughCantAssociateThroughHasOneOrManyReflection error raises.
For more details, see r7062.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@9016 e93f8b46-1217-0410-a6f0-8f06a7374b81