I dont think it is possible to perform this without cloning one repo locally. However you can do the following:
Code:
git clone https://github.com/cosmin/git-hg.git
cd git-hg
git remote add offbytwo https://github.com/offbytwo/git-hg.git
git fetch offbytwo
git diff offbytwo/master
This will show you the difference between the master branch cloned from cosmin and the master branch from offbytwo. I would also suggest to use something like qgit for a nice visual representation of the branch trees and so on. This will quickly show you the common ancestor and so on.