Merge branch 'wk/submodule-update-remote'

The beginning of 'integrate with the tip of the remote branch, not the commit recorded in the superproject gitlink' support. * wk/submodule-update-remote: submodule add: If --branch is given, record it in .gitmodules submodule update: add --remote for submodule's upstream changes submodule: add get_submodule_config helper funtion