The bulk of "git submodule foreach" has been rewritten in C.

* pc/submodule-helper-foreach:
  submodule: port submodule subcommand 'foreach' from shell to C
  submodule foreach: document variable '$displaypath'
  submodule foreach: document '$sm_path' instead of '$path'
  submodule foreach: correct '$path' in nested submodules from a subdirectory