Function get_closest_upstream_commit

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

Returns the most recent (ordered chronologically) commit found in the local history that should exist upstream. We identify upstream commits by the e-mail of the commit author.

If we are in CI, we simply return our first parent.