Function pinned_diff 

pub fn pinned_diff(
    vcs: &dyn Vcs,
    source: &ReviewSource,
    pr_head: Option<(&str, &str)>,
) -> Result<DiffModel, VcsError>

The diff for a commit, range or PR source. pr_head is the PR’s (merge_base, head); None rejects the PR so it never reads as unchanged. Other sources read back empty.