wolfibump doesn't work with different fork repo name

Description

Transferring issue over:

I was trying to use wolfibump with my wolfi-dev/os fork wlynch/wolfi-os, and ran into problems.

There's a lot of code that assumes the default forks name of a repositories.

Currently you get an error like:

2024/02/21 14:22:04 unable to create a pull request: will ignore this execution: branch already exists: while checking if repository is a fork of wolfi-dev/os: checking if repository is a fork: getting repository: GET https://api.github.com/repos/wlynch/os: 404 Not Found []

I tried wiring up a fork repo name flag, which then led me to the error:

2024/02/21 14:39:31 unable to create a pull request: will ignore this execution: branch already exists: a branch named main already exists in wlynch/wolfi-os

which doesn't quite make sense since we'd expect the main branch to exist on all forks.

I then tried setting the --wolfi-packages-repo-branch=wolfibump flag, but that failed because it assumes the same branch name for the fork checkout.

2024/02/21 14:43:15 failed to clone repository https://github.com/wolfi-dev/os.git into /var/folders/83/j7crs0zj5g9_nj3wb9hql9hh0000gn/T/wolfibump410073369: reference not found

Internal

Start by tracing wolfibump's repository and branch handling around the fork repository name and the --wolfi-packages-repo-branch flag. Reproduce the failures with a fork named wlynch/wolfi-os and inspect how the main and wolfibump branches are selected. Done means wolfibump can use a fork with a different repository name and distinct branch names without assuming wolfi-dev/os defaults.