Rails3 compatibility: scm: darcs: fix error of test_fetch_changesets_from_scratch at unit model test

On Rails 3.1, following error raises.

<pre>
test_fetch_changesets_from_scratch(RepositoryGitTest):
ActiveRecord::HasManyThroughCantAssociateThroughHasOneOrManyReflection:
 Cannot modify association 'Repository::Git#changes'
 because the source reflection class 'Change' is associated to 'Changeset' via :has_many.
    test/unit/repository_git_test.rb:68:in `test_fetch_changesets_from_scratch'
</pre>