Compare unrelated remote branches

[dirkmc]

Presently we allow comparisons between branches on the same remote, eg /repo/QmU1HJJDFSM8JJq4r31wSLfj51oysQCswz7aL78UWZHuMC/compare/master...feat%2Fcoreapi%2Fswarm It would be great if we allowed unrelated remote names here, eg /repo/<cid>/compare/master...ipns://QmRepo/feature