anchor.diff:          --rev="$commit_sha"
     '';
   
  -in [ updateScript url ]
  +in [
  +  updateScript
  +  "--url=${builtins.toString url}"
  +] ++ lib.optionals (branch != null) [
  +  "--branch=${branch}"
  +]
   
  
positive.code:   updateScript = writeShellScript "unstable-update-script.sh" ''
      set -ex
  
      url="$1"
  
      # By default we set url to src.url
      if [[ -z "$url" ]]; then
  
negative.code: 
      # Get info about HEAD from a shallow git clone
      tmpdir="$(${coreutils}/bin/mktemp -d)"
  
      cloneArgs=(
        --bare
        --depth=1
      )
  
      if [[ -n "$branch" ]]; then
          cloneArgs+=(--branch="$branch")
      fi
  
      ${git}/bin/git clone "''${cloneArgs[@]}" "$url" "$tmpdir"
  
      pushd "$tmpdir"
      commit_date="$(${git}/bin/git show -s --pretty='format:%cs')"
      commit_sha="$(${git}/bin/git show -s --pretty='format:%H')"
  