Function is_ancestor 

pub fn is_ancestor(
    repo: &Repository,
    ancestor: Sha,
    descendant: Sha,
) -> Result<bool, GitError>

Return true iff ancestor is an ancestor of descendant (or equals it).

Uses the merge_base(A, B) == A identity. A commit is its own ancestor; unrelated commits return false; missing commits propagate as GitError.

Errors

Returns GitError::MergeBase if the merge-base computation fails.