Added support for git submodules

[JordanReiter]

No description provided.

[carljm]

Thanks for the pull request! I think we had discussed this earlier and come down on the side of not supporting submodules, but I'm no longer recalling the reasons for that. @jezdez, do you remember?

In any case, this would need tests.

[jezdez]

I think we basically didn't want to complicate the VCS implementation further by supporting specific features of some VCS but instead only the bare minimum for getting a repo. I strongly believe this rule applies to git submodules, too.

[JordanReiter]

Is this explicit anywhere in the docs? Might be a good idea in case anyone attempts to set up a git-based install that depends on other git submodules. I didn't find out until after I tried to get it to work...

[carljm]

Yeah, I guess we've had enough trouble keeping the git backend working consistently even for the simple features, we don't really need the burden of supporting more complex features. I've added a note to the docs to clarify this; sorry @JordanReiter for the wasted time and thanks for the contribution.

[carljm]

#289 is the original issue with discussion of this.

[carljm]

This was fixed in merging #577.