Rails3: scm: git: fix error of test_fetch_changesets_from_scratch at unit model test

Rails3: scm: git: fix error of test_fetch_changesets_from_scratch at unit model test

On Rails 3.1, following error raises.

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'

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@7062 e93f8b46-1217-0410-a6f0-8f06a7374b81