Specify branch for retrieving snakemake repository

Currently, the `git clone ...` command in the projects.py file retrieves a single branch, but it's always the default. Adding a `-b {branchname}` option there, and a config value to set it, could add some flexibility. Adding an issue here because I know I'm going to forget to account for the fact that if the user _doesn't_ specify a branch, we don't know if it should be `main` or `master` or something else by default, so if `branchname` is unset, we shouldn't include that option at all.