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 #80 that is
spun out of #106.

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