The previous commit fixed the problem that the staged but that ignored
submodules did not show up in the status output of the commit command and
weren't committed afterwards either. But when commit doesn't generate the
status output (e.g. when used in a script with '-m') the ignored submodule
will still not be committed. This is because in that case a different code
path is taken which calls index_differs_from() instead of calling the
wt_status functions.

Fix that by calling index_differs_from() from builtin/commit.c with a
diff_options argument value that tells it not ignore any submodule changes
unless the '--ignore-submodules' option is used. Even though this option
isn't yet implemented for cmd_commit() but only for cmd_status() this
prepares cmd_commit() to correctly handle the '--ignore-submodules' option
later. As status and commit share the same ignore_submodule_arg variable
this makes the code more robust against accidental breakage and documents
how to correctly call index_differs_from().

Change the expected result of the test documenting this problem from
failure to success.
Signed-off-by: NJens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>