Set value of source variables when they are defined

I am combining helm-ls-git sources with other sources, e.g.

```lisp
(defun c/helm-jump ()
  (interactive)
  (helm :sources '(helm-source-buffers-list
                   helm-source-ls-git
                   helm-source-recentf
                   helm-source-buffer-not-found)))
```

At the moment, the helm-ls-git sources are initialised to nil on start up. Only after calling `helm-ls-git-ls` the sources are initialised with their proper value.

To be able to use the sources without calling `helm-ls-git-ls` first, I refactored the code so that the proper value of the sources is set when they are first defined.