OperateOnRepos acts as a switch, depending upon whether the user provided an explicit list of repos to operate.

We process all work that can be done up to the open pull request API call in parallel However, we then separately process all open pull request jobs, through the PRChan.

RepoSelection is a struct that presents a uniform interface to present to OperateRepos that converts user-supplied repos in the format of <org-name>/<repo-name> to GitHub API response objects that we actually pass into processRepos which does the git cloning, command execution, committing and pull request opening.