Is there a way to fetch commits that are detached

First some context: I'm trying to use the pre-receive hook to run some tests
on a Hudson server before accepting the updates to a branch. At the moment I'm
trying to have the Hudson job clone the repository (using the "git" protocol)
and checkout the new ref as a detached head e.g.

$ git clone $GIT_REPOSITORY repository
$ cd repository
$ git checkout $GIT_COMMIT

But the checkout fails, apparently because the commit object hasn't been
pulled into the local repository. Only a pack file is present in the local
repository's objects directory which presumably doesn't have the commit.

Is there a way to explicitly fetch whatever objects are necessary to checkout
the commit?

Would I be better off using git-archive to tar up a copy of the source
instead, or some other means of copying the codebase over for testing?

Thanks,
Allan