Function start_merge 

pub fn start_merge(
    dir: &Path,
    gd: &Path,
    target: &str,
    tip: &str,
    out: &mut dyn Write,
) -> Result<Result<(), &'static str>, Box<dyn Error>>

Start land’s own merge of tip (git merge --no-commit --no-ff), recorded in <git-dir>/weave-land-merge as in-flight first. Ok(Err) (the reason; the report printed) when git did not complete the merge — it could not write the index, say — in which case whatever it left is aborted: a merge git did not finish is never judged as a merge.