Merge branch 'bw/push-options-recursively-to-submodules' into maint

"git push --recurse-submodules $there HEAD:$target" was not
propagated down to the submodules, but now it is.

* bw/push-options-recursively-to-submodules:
  submodule--helper: teach push-check to handle HEAD

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

diff --cc submodule.c
Simple merge

diff --cc t/t5531-deep-submodule-push.sh
Simple merge