Merge branch 'sb/submodule-init'

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