message: fix mem replace unused
diff: @@ -838,7 +838,7 @@ fn maybe_gc_repo(repo: &mut git2::Repository) -> CargoResult<()> {
  );
  if out.status.success() {
  let new = git2::Repository::open(repo.path())?;
  - mem::replace(repo, new);
  + let _ = mem::replace(repo, new);
  return Ok(());
  }
  }
  