Function update 

fn update<G: GitHistory + ProposalGit>(
    git: &G,
    worktree: &Path,
    request: &BranchSync<'_>,
    authority_head: &str,
    branch_head: Option<&str>,
    apply: impl FnOnce(&Path, &Committer<'_, G>) -> Result<(), ProposalError>,
) -> Result<String, ProposalError>

Apply the edit on the tip, commit, and push unless the branch already holds an equivalent proposal. In automatic mode, any branch the marker does not vouch for is refused.