Support cherry-pick from a different repository

[gastaldi]

The problem I'm trying to solve:

Having two repositories (eg. https://github.com/quarkusio/quarkus-http/ and https://github.com/undertow-io/undertow), I'm trying to cherry-pick some fixes from the latter to the former repository (eg. undertow-io/undertow@c81583a (#1551))

How I imagine hub could expose this functionality:

I'd expect running the following command in a cloned https://github.com/quarkusio/quarkus-http would work:

hub cherry-pick https://github.com/undertow-io/undertow/pull/1551/commits/c81583ac096f9d0cae024e0282ba2c1362c9643c

But if fails with fatal: couldn't find remote ref refs/pull/1551/head