This would be useful for the IPFS bridge, and seems good to complete the
API in any sense.
On the other hand, I don't think this test is correct at all. `Content`
doesn't derive from `HashableObject` like the others.Besides that, I suspect that is
why `content_git_object` didn't already exist.
I get the sense there are number of historical subtleties here I am
probably stomping over in ignorance :)it seems good to complete the API too.