Initialize helm-source-ls-git

Hi

I'd like to first thank you for the great package, this is one of my favorite helm packages.

The way I'm using helm-ls-git is adding helm-source-ls-git to helm-for-files-preferred-list, and invoking helm-for-files.
The problem is that helm-source-ls-git is initially nil, and helm-for-files doesn't show git files.
Invoking helm-ls-git-ls once, helm-source-ls-git is initialized and helm-for-files shows git files as expected.
I am getting around this by doing this in my init.el.

```
(setq helm-source-ls-git-status
(and (memq 'helm-source-ls-git-status helm-ls-git-default-sources)
(helm-make-source "Git status" 'helm-ls-git-status-source
:fuzzy-match helm-ls-git-fuzzy-match))
helm-source-ls-git
(and (memq 'helm-source-ls-git helm-ls-git-default-sources)
(helm-make-source "Git files" 'helm-ls-git-source
:fuzzy-match helm-ls-git-fuzzy-match))
helm-source-ls-git-buffers
(and (memq 'helm-source-ls-git-buffers helm-ls-git-default-sources)
(helm-make-source "Buffers in git project" 'helm-source-buffers
:header-name #'helm-ls-git-header-name
:buffer-list (lambda () (helm-browse-project-get-buffers
(helm-ls-git-root-dir)))
:keymap helm-ls-git-buffer-map)))
```

I suppose this should happen on helm-ls-git side.