This does two things:
* add --recursive option to git clone command in clone(). This will
initialize all submodules when cloning a remote repository.
* Add submodule_update() and call that from fetch(). submodule_update()
calls two git commands iff the file .gitmodules exists in the
repository:
* 'git submodule sync' - synchronizes the submodules' remote URL
configuration setting to the value in .gitmodules.
* 'git submodule update --init --recursive' - initialize and update
registered submodules to the commit specified in the index of the
containing repository.
If a repository was cloned without --recursive, submodule_update() will
ensure that the submodules are initialized and updated.
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