Ignore submodules running `git status`

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs? | no
| Applies to    | 
| Fixed tickets | 

When doing

```bash
symfony-docs$ git st
On branch master
Your branch is up-to-date with 'origin/master'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)
  (commit or discard the untracked or modified content in submodules)

	modified:   _exts (untracked content)

no changes added to commit (use "git add" and/or "git commit -a")
```

the directory `_exts` should not get listed.

The patch is based on http://stackoverflow.com/questions/5127178/gitignore-files-added-inside-git-submodules ... ready that issue there is another option `dirty`.

Not sure what's best so:

- [ ] choose between `dirty` and `untracked`