Use default branch of repository if none is specified
If no branch is explicitly specified treep currently defaults to using "master". This causes problems for repositories that do not have a master branch (e.g. because they switched to "main" or use some completely different scheme).
To fix this, treep should by default use the default branch that is provided by the repository (i.e. what you get when only calling git clone repo-url
and only checkout a specific branch if one is explicitly stated in the configuration.