submodule summary: ignore --for-status option

submodule summary: ignore --for-status option

The --for-status option was an undocumented option used only by
wt-status.c, which inserted a header and commented out the output. We can
achieve the same result within wt-status.c, without polluting the
submodule command-line options.

This will make it easier to disable the comments from wt-status.c later.

The --for-status is kept so that another topic in flight
(bc/submodule-status-ignored) can continue relying on it, although it is
currently a no-op.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>