anchor.diff:  
       # Get info about HEAD from a shallow git clone
       tmpdir="$(${coreutils}/bin/mktemp -d)"
  -    ${git}/bin/git clone --bare --depth=1 "$url" "$tmpdir"
  +    ${git}/bin/git clone --bare --depth=1 \
  +      ${lib.optionalString (branch != null) "--branch ${branch}"} \
  +      "$url" "$tmpdir"
       pushd "$tmpdir"
       commit_date="$(${git}/bin/git show -s --pretty='format:%cs')"
       commit_sha="$(${git}/bin/git show -s --pretty='format:%H')"
  
positive.code: # This is an updater for unstable packages that should always use the latest
  # commit.
  { url ? null # The git url, if empty it will be set to src.url
  }:
  
  let
  
negative.code: { lib
  , writeShellScript
  , coreutils
  , git
  , nix
  