Make Grit::Status ignore submodule-contained files

Submodule files were showing as untracked in the Grit::Status, which is a bit of a problem (to me at least).
Next is probably going to be taking ignored files out (or flagging them appropriately).

I also took the opportunity to fix one test case that was failing on Xcode-provided git binaries, because they include a custom version tag, e.g. my git's git --version reads "git version 1.7.7.5 (Apple Git-26)"