Allow tracking submodules.

Git>=1.8.2 allows the ability for submodules to track the tip of a branch
versus using just a single SHA-1 for the submodule, and this commit allows
that for the CliGitAPIImpl.  JGitAPIImpl just gets a "not implemented".
Test case for updating a tracking submodule
Merge branch 'master-add-tracking-submodules' into master-work