"git push --recurse-submodules --push-option=<string>" learned to
propagate the push option recursively down to pushes in submodules.

* bw/push-options-recursively-to-submodules:
  push: propagate remote and refspec with --recurse-submodules
  submodule--helper: add push-check subcommand
  remote: expose parse_push_refspec function
  push: propagate push-options with --recurse-submodules
  push: unmark a local variable as static