git-gui: 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 git-gui. 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
"Staged Changes (Will Commit)".

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: Pat Thoyts <patthoyts@users.sourceforge.net>

lib/diff.tcl

@@ -287,6 +287,9 @@ proc start_show_diff {cont_info {add_opts {}}} {
 	if {$w eq $ui_index} {
 		lappend cmd diff-index
 		lappend cmd --cached
+		if {[git-version >= "1.7.2"]} {
+			lappend cmd --ignore-submodules=dirty
+		}
 	} elseif {$w eq $ui_workdir} {
 		if {[string first {U} $m] >= 0} {
 			lappend cmd diff