git submodule: ignore dirty submodules for summary and status

git submodule: ignore dirty submodules for summary and status

The summary and status commands only care about submodule commits, so it is
rather pointless that they check for dirty work trees. This saves the time
needed to scan the submodules work tree. Even "git status" profits from these
savings when the status.submodulesummary config option is set, as this lead to
traversing the submodule work trees twice, once for status and once again for
the submodule summary. And if the submodule was just dirty, submodule summary
produced rather meaningless output anyway:

 * sub 1234567...1234567 (0):

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>