[ -e "$GIT_DIR/refs/heads/$newbranch" ] &&
die "git checkout: branch $newbranch already exists"
git-check-ref-format "heads/$newbranch" ||
[ -e "$GIT_DIR/refs/heads/$newbranch" ] &&
die "git checkout: branch $newbranch already exists"
git-check-ref-format "heads/$newbranch" ||