Merge branch 'sb/submodule-rm-absorb'

Merge branch 'sb/submodule-rm-absorb'

"git rm" used to refuse to remove a submodule when it has its own
git repository embedded in its working tree.  It learned to move
the repository away to $GIT_DIR/modules/ of the superproject
instead, and allow the submodule to be deleted (as long as there
will be no loss of local modifications, that is).

* sb/submodule-rm-absorb:
  rm: absorb a submodules git dir before deletion
  submodule: rename and add flags to ok_to_remove_submodule
  submodule: modernize ok_to_remove_submodule to use argv_array
  submodule.h: add extern keyword to functions