gitk: Show staged submodules regardless of ignore config

gitk: Show staged submodules regardless of ignore config

Currently setting submodule.<name>.ignore and/or diff.ignoreSubmodules to
"all" suppresses all output of submodule changes for gitk. This is really
confusing, as even when the user chooses to record a new commit for an
ignored submodule by adding it manually this change won't show up under
"Local changes checked in to index but not committed".

Fix that by using the '--ignore-submodules=dirty' option for both callers
of "git diff-index --cached" when the underlying git version supports that
option.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>