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

diff --cc Documentation/config.txt
Simple merge