Added libgit2 submodule to Julia

I'm currently working on issue #4158 for reimplementing Git and started by adding libgit2 as a submodule.  It currently is able to clone the repo and install itself properly in `JL_LIBDIR` and remove itself upon cleaning.  A code review on this though would be super appreciated!  I want to make sure there aren't any particular conventions I'm missing.

Also, any suggestions on the next best step to move forward are also welcomed!  (the plan is to start writing the C api calls for libgit2 in julia/src)