Implement convenience function for submodule update

This is a PR for the work in progress to include a convenience function for submodule update functionality.

The 1st and 2nd commits bring relative URL paths in gitmodules in line with core git.

The 3rd commit introduces a `git_submodule_do_update` function. One place in particular I was looking for thoughts was the `git_submodule_update_options` struct. There are several ways this could be done. Initially, I have chosen to just expose the properties that I think might make sense on this struct. Also, there is already a function named `git_submodule_update` that is a getter for a config setting, which feels out of place with `git_submodule_sync` and `git_submodule_init` - it seems this should be the function for performing a submodule update.

It also seems that there are several functions that are much less useful after the updates to the clone API - in particular `git_submodule_add_setup`, `git_submodule_add_finalize`, and `git_submodule_repo_init`. These set up / finalize a series of steps where you can setup a repository and then clone into it, but the `clone_into` function is no longer exposed. Is there a reason for `git_submodule_add_setup` to exist? or should these be replaced with a more appropriate API for adding a submodule?
HEAD here, too.