If it's a copyright violation (be sure that it ACTUALLY is!) they will remove content in response to a DMCA request, but any forks will only be removed if you manually find them and issue a request for each fork. This isn't very useful if you accidentally uploaded your own copyrighted material though, since that's not a violation you could issue a notice for.

Can you DMCA yourself for someone else's copyrighted material? That's what I'm talking about here.

You have to be the copyright holder or their representative, so no, it would technically be illegal to DMCA yourself for violating someone else's IP. If you asked github support nicely they might help, though.

There isn't, you need to contact them so they can delete the offending objects.

I don't think there's a need to erase copyrighted material? If it's your material then the copyright still holds. If it's not your material it's a problem between GitHub and the copyright holder who can DMCA the "hidden commit" if for some godforsaken reason the copyright holder somehow found the commit and cares.