Refactor dirty submodule detection in diff-lib.c

Refactor dirty submodule detection in diff-lib.c

Moving duplicated code into the new function match_stat_with_submodule().
Replacing the implicit activation of detailed checks for the dirtiness of
submodules when DIFF_FORMAT_PATCH was selected with explicitly setting
the recently added DIFF_OPT_DIRTY_SUBMODULES option in diff_setup_done().

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