Function already_in 

pub fn already_in(
    repo: &Path,
    upstream: &str,
    head: &str,
) -> Result<bool, String>

Whether every commit of head is already contained in upstream by patch identity. git cherry marks a commit - when an equivalent change (same git patch-id --stable) is upstream, + when it is not; no + lines — including the empty output of a plain ancestor — means there is nothing left to merge.