allow crawl to use a "source-of-truth" branch other than main

[cmwylie19]

Describe what should be investigated or refactored

Crawl compares new PRs against a source of truth branch. In GitHub that is typically main, but this branch should be configurable.

[mjnagel]

I think with octokit we should be able to get the repo and pull the default_branch property from that for the default. Could also make a CLI param for branch/tag I suppose but reading the default branch would cover most needs imo.