Function git_upstream_merge_base

fn git_upstream_merge_base(
    config: &GitConfig<'_>,
    git_dir: Option<&Path>,
) -> Result<String, String>

Finds the nearest merge commit by comparing the local HEAD with the upstream branch’s state. To work correctly, the upstream remote must be properly configured using git remote add <name> <url>. In most cases get_closest_merge_commit is the function you are looking for as it doesn’t require remote to be configured.