I'm sure git rev-list will do what you want:
git rev-list --no-walk <commit> --objects
See the git rev-list documentation at http://www.kernel.org/pub/software/scm/git/docs/git-rev-list.html for more information.
Edit: Added --no-walk as proposed by Pailo Eberman; it will show only objects for a given ref, without ancestors.
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