Update of "git submodule" to move pieces of logic to C continues.
* sb/submodule-init:
submodule init: redirect stdout to stderr
submodule--helper update-clone: abort gracefully on missing .gitmodules
submodule init: fail gracefully with a missing .gitmodules file
submodule: port init from shell to C
submodule: port resolve_relative_url from shell to C