Selectively redirect submodules to the cached repo

If Kochiku is asked to build a branch that contains a new submodule,
that submodule will not be present inside the original cached repo.
Handle this case by only redirecting the submodules that are present in
the cached repo and perform a full clone of the rest.
Merge pull request #26 from square/rob/smarter-submodules

Selectively redirect submodules to the cached repo

R: jkingdon