How to remove a commit object?

I maintain a shared repository a bunch of my coworkers push to for code reviews. It has accumulated a lot of packfiles, so I want to shrink it down a bit, but there's a problem:

% git repack -A -d Counting objects: ... error: Could not read 125bf191b65189aaec7a6aa24ff26460d141d587 fatal: bad tree object 125bf191b65189aaec7a6aa24ff26460d141d587

"git fsck" confirms that the tree object is missing:

% git fsck
broken link from  commit 1b2f0595bb4a6c2e17ca43a9cc41feec88c72a47
               to    tree 125bf191b65189aaec7a6aa24ff26460d141d587
...
missing tree 125bf191b65189aaec7a6aa24ff26460d141d587

This is a dangling commit, but that's fine; for this particular repository we actually *want* lots of dangling commits since they represent the history of people's code review requests. (Hence me running git-repack with -A instead of -a.)

Given that it's dangling, it seems like it'd be safe to just remove entirely (we lose that little bit of code-review history but we've lost it already anyway with the tree object missing). But I'm not sure how to do it. Is it possible to delete a commit object, and if so, how?

I don't know how the corruption happened in the first place. There was a short time at one point where the permissions on the object directories were inconsistent, so it's possible someone pushed during that period and managed to create the commit object file in .git/ objects but didn't have permission to create the tree object. That's just speculation on my part, though. This is the only corrupt object in the repository according to git-fsck, so at this point I just want to know how to get rid of it so I can do the repack.

Thanks!

-Steve