Merge branch 'sb/submodule-deinit-all' into maint

Correct faulty recommendation to use "git submodule deinit ." when
de-initialising all submodules, which would result in a strange
error message in a pathological corner case.

* sb/submodule-deinit-all:
  submodule deinit: require '--all' instead of '.' for all submodules

diff --cc git-submodule.sh
Simple merge

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