Function base_ref_for_parents 

pub fn base_ref_for_parents(parents: &[String]) -> String

The diff base for a commit given its structured parent hashes: the first parent, or the empty tree when parentless. Prefer this over base_ref_for_commit_parents when the parents are already a slice — it avoids a join/split round-trip. The string variant remains the port shim for the CLI-output path that receives space-separated parents.