Obliterating a commit

[Wincent Colaiuta <win@wincent.com>]

A couple of days ago I mistakenly checked in a file that had some  
confidential information in it. I immediately realized and amended my  
commit, and this is a local repository whose contents won't be  
visible until I push them out.

So how do I *really* get rid of the that commit before publishing? I  
couldn't find any porcelain or plumbing to do this. Do I have to  
manually destroy it? ie. wind back the HEAD, manually remove the  
commit object, the corresponding tree object, the corresponding file  
blobs, and probably manually remove the entry from the reflog as well?

Is there a "shortcut" wherein I can somehow mark this commit and its  
related tree and file blobs as unreachable, and then use git-prune to  
erradicate them?

Cheers,
Wincent