checkout: Add test for diff.ignoreSubmodules

While at it, document that checkout uses this flag too in the Documentation.
Signed-off-by: NJens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -829,7 +829,8 @@ diff.renames::
 diff.ignoreSubmodules::
 	Sets the default value of --ignore-submodules. Note that this
 	affects only 'git diff' Porcelain, and not lower level 'diff'
-	commands such as 'git diff-files'.
+	commands such as 'git diff-files'. 'git checkout' also honors
+	this setting when reporting uncommitted changes.
 
 diff.suppressBlankEmpty::
 	A boolean to inhibit the standard behavior of printing a space

t/t2013-checkout-submodule.sh

@@ -39,4 +39,11 @@ test_expect_success '"checkout <submodule>" updates the index only' '
 	git diff-files --quiet
 '
 
+test_expect_success '"checkout <submodule>" honors diff.ignoreSubmodules' '
+	git config diff.ignoreSubmodules dirty &&
+	echo x> submodule/untracked &&
+	git checkout HEAD >actual 2>&1 &&
+	! test -s actual
+'
+
 test_done