Reverts #128

It looks like shallow clones are a lot more hairy than we realized.
We discovered the following:

1. We currently have no way to figure out if a clone is shallow or not.
   An option has been added to rev-parse (why rev-parse?!) but that's
   too new and not in any packaged git.
2. If your repo has under 50 commits, it will not be shallow!
3. If you try to run git fetch --unshallow on a non-shallow repo it is a fatal error