Merge branch 'jc/submodule-add' into next

"git submodule add" to add a submodule under <name> segfaulted,
when a submodule.<name>.something is already in .gitmodules file
without defining where its submodule.<name>.path is, which has been
corrected.

* jc/submodule-add:
  submodule add: sanity check existing .gitmodules

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

diff --cc t/t7400-submodule-basic.sh
Simple merge