Repositories directory should be configurable

[tplass-ias]

My org has > 1k repositories, and I already keep them all cloned locally for various searching. I would like to leverage my existing directory of cloned repos instead of cloning fresh.

This tool already does various repo and branch cleanup, so it should be mostly ok to point at pre-cloned repos 🤷 I think this can be achieved by providing a config value to override the repos_dir property on WorkDir with an absolute path.

If this seems like a feature you would like for this project, I can author a PR :)