class Rugged::SubmoduleCollection

Returns the newly created +submodule+

directly in workdir.
gitlink to the repository in +.git/modules+ vs. repository
(defaults to +true+) should workdir contain a
:gitlink ::
and the following:
The +options+ hash accepts all options supported by Rugged::Remote#fetch

- +path+: path at which the submodule should be created
- +url+: URL for the submodule's remote

submodule contents.
This does "git submodule add" incuding fetch and checkout of the

Add a new +submodule+.

submodules.setup_add(url, path[, options]) -> submodule
call-seq:

def add(url, path, options = {})
  submodule = setup_add(url, path, options)
  clone_submodule(submodule.repository, options)
  submodule.finalize_add
end

3. checkouts the submodule
2. sets up a master branch to be tracking origin/master
1. fetches the remote
This provides a ghetto clone implementation that:

requires the target for the clone to be an empty dir.
with a workdir for the submodule. libgit2's `git_clone` however
currently libgit2's `git_submodule_add_setup` initializes a repo

def clone_submodule(repo, fetch_options)
  # the remote was just added by setup_add, no need to check presence
  repo.remotes['origin'].fetch(fetch_options)
  repo.branches.create('master','origin/master')
  repo.branches['master'].upstream = repo.branches['origin/master']
  repo.checkout_head(strategy: :force)
end