cross-fork comparisons

Howdy,
I'm using this change to allow cross-fork comparisons.

In my particular use case, I want to compare by base, and suggest updates through a specially crafted pull request, downstream to my forkers.  In order to do that, I need to specify their head as the base, and mine as the head.

Github does `base:branch...head:branch` for their syntax.  From their docs, they optionally even allow `user/base:branch...user/head:branch` which might even help with non-
forked" forks.