The `Git` builder should fetch submodules

[tailhook]

Let's make it fetch submodules by default, and allow to disable it.

This should be done before 0.6, so we don't introduce incompatibilities in a minor release.

/cc @lilianmoraru

[tailhook]

Well, because the Git command is there not just to do git clone, but also for caching, it's harder than I was thinking of. I mean we clone with --bare to cache, then clone from cache. And bare clone doesn't have --recursive option. So we probably need to traverse submodules manually.

So we can't do this in 0.6 milestone.