Use a template that directly includes the repository name and the branch
checked out. This makes finding and deleting the worktree directory if
anything goes wrong a lot easier.
Checkout the worktree directly in the temporary directory. With that
`TEMP_DIR` becomes obsolete.
Fix the exit condition when the function is called outside of a git
repository.
Use zsh's `:t` History Expansion Modifier, as `$?` does not keep `git
rev-parse`'s exit code, but rather `basename`'s.
