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.
Split `toplevel` definition and assignment as the `local` builtin has
it's own exit code. Thus the function previously did not return if the
`PWD` was not in a git repo.
Some distributions ship older versions of `column` that do not have the
flags `-dNLO`. This is a workaround around this limitation that creates
the same output using other tools.
Move `git-track` into its own autoloadable function.
This way it is
better maintainable as in one-liner format and brings the possibility of
having more complexity (e.g. for better portability).
By that it is also now executable as `git-track` as well as `git track`
as before.
