Encapsulate repository fields in config

[joneshf]

We want to change these fields to be smarter in the future.

These fields are all fine for the case where there's only an origin repo that is being worked against. Once we get to wanting to support an upstream repo for a fork, these fields start to take on a bit more logic. In an effort to make that step a bit easier, we encapuslate the direct access to these fields behind functions where that logic can exist.

This is pre-work for https://github.com/getcord/spr/issues/80 that is spun out of https://github.com/getcord/spr/pull/106.

Test Plan: There should be no actual changes here. It should be enough to run cargo test and see no failures.