Function prepare 

pub fn prepare(
    dir: &Path,
    opts: &Options,
    host: &Host,
    out: &mut dyn Write,
    tip: &str,
    env: &[(&str, &str)],
) -> Result<Prep, Box<dyn Error>>

Steps 2–4 of the loop against one fetched tip, on the branch checked out in dir: merge tip in and gate every file of the merge, gate every other unchecked merge on the branch, run the verify command on the final tree. Ready(HEAD) when all of it passed — the caller publishes HEAD with a lease on tip. env is extra environment for the verify command.

A merge refusal leaves the merge in progress in dir, the refused files conflicted, so the person can resolve them in place.