gitfs: support git submodules

[0x450x6c]

Hello.

I use git submodule as a store, and gopass didn't recognize it as a git repo.

With this change gopass can recognize git submodules as a repository, so gopass sync works. (tested locally)

[dominikschulz]

Oh my, I don't like git submodules. The UX is terrible. Let me see how much complexity this would add ...

[dominikschulz]

Sorry for the long delay. The code itself looks fine, but we need two things before we can merge it:

1. Comments, explaining what is happening. I'm not deeply familiar with submodules and some explanation is necessary to make sense of this.
2. Tests, showing and validating the implementation.