Merge branch 'sb/submodule-update-reset-fix' into maint

When resetting the working tree files recursively, the working tree
of submodules are now also reset to match.

* sb/submodule-update-reset-fix:
  submodule: submodule_move_head omits old argument in forced case
  unpack-trees: oneway_merge to update submodules
  t/lib-submodule-update.sh: fix test ignoring ignored files in submodules
  t/lib-submodule-update.sh: clarify test