Merge branch 'sb/submodule-recommend-shallowness'

An upstream project can make a recommendation to shallowly clone
some submodules in the .gitmodules file it ships.

* sb/submodule-recommend-shallowness:
  submodule update: learn `--[no-]recommend-shallow` option
  submodule-config: keep shallow recommendation around

diff --cc builtin/submodule--helper.c
Simple merge

diff --cc git-submodule.sh
Simple merge