Allow configuring upstream repository for pull

[Kobzol]

This is useful e.g. for things like rust-lang/rust#143412. We want to do some non-trivial subtree change in rust-lang/rust, and we want to test what it will do on the subtree's CI before we actually merge that change.

So with this, we can do:

1. Make the change in your fork of rust-lang/rust, push it to its master branch
2. Do a rustc-josh-sync pull --upstream <username>/rust on the subtree
3. Create a draft subtree PR from that change and check if CI is green