submodule: check for unstaged .gitmodules outside of config parsing

submodule: check for unstaged .gitmodules outside of config parsing

Teach 'is_staging_gitmodules_ok()' to be able to determine in the
'.gitmodules' file has unstaged changes based on the passed in index
instead of relying on a global variable which is set during the
submodule-config parsing.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>