Function derive 

pub fn derive(
    repo: &Repository,
    branches: &[String],
    config: &DeriveConfig,
) -> Result<Graph>

Derive a Toolpath Graph from the given repository and branch names.

Branch strings are parsed as BranchSpecs (supporting "name:start" syntax). A single branch yields a single-path graph (one entry in paths); multiple branches yield a multi-path graph.