message: remove mem replace instead
diff: @@ -11,7 +11,6 @@ use serde::Serialize;
  use std::env;
  use std::fmt;
  use std::fs::File;
  -use std::mem;
  use std::path::{Path, PathBuf};
  use std::process::Command;
  use url::Url;
  @@ -838,7 +837,7 @@ fn maybe_gc_repo(repo: &mut git2::Repository) -> CargoResult<()> {
  );
  if out.status.success() {
  let new = git2::Repository::open(repo.path())?;
  - let _ = mem::replace(repo, new);
  + *repo = new;
  return Ok(());
  }
  }
  