git status: ignoring untracked files must apply to submodules too

git status: ignoring untracked files must apply to submodules too

Since 1.7.0 submodules are considered dirty when they contain untracked
files. But when git status is called with the "-uno" option, the user
asked to ignore untracked files, so they must be ignored in submodules
too. To achieve this, the new flag DIFF_OPT_IGNORE_UNTRACKED_IN_SUBMODULES
is introduced.

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