Fix 'git checkout <submodule>' to update the index

While 'git checkout <submodule>' should not update the submodule's
working directory, it should update the index.  This is in line with
how submodules are handled in the rest of Git.

While at it, test 'git reset [<commit>] <submodule>', too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-checkout.c

@@ -53,9 +53,6 @@ static int update_some(const unsigned char *sha1, const char *base, int baselen,
 	int len;
 	struct cache_entry *ce;
 
-	if (S_ISGITLINK(mode))
-		return 0;
-
 	if (S_ISDIR(mode))
 		return READ_TREE_RECURSIVE;
 

t/t2013-checkout-submodule.sh

@@ -0,0 +1,42 @@
+#!/bin/sh
+
+test_description='checkout can handle submodules'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	mkdir submodule &&
+	(cd submodule &&
+	 git init &&
+	 test_commit first) &&
+	git add submodule &&
+	test_tick &&
+	git commit -m superproject &&
+	(cd submodule &&
+	 test_commit second) &&
+	git add submodule &&
+	test_tick &&
+	git commit -m updated.superproject
+'
+
+test_expect_success '"reset <submodule>" updates the index' '
+	git update-index --refresh &&
+	git diff-files --quiet &&
+	git diff-index --quiet --cached HEAD &&
+	test_must_fail git reset HEAD^ submodule &&
+	test_must_fail git diff-files --quiet &&
+	git reset submodule &&
+	git diff-files --quiet
+'
+
+test_expect_success '"checkout <submodule>" updates the index only' '
+	git update-index --refresh &&
+	git diff-files --quiet &&
+	git diff-index --quiet --cached HEAD &&
+	git checkout HEAD^ submodule &&
+	test_must_fail git diff-files --quiet &&
+	git checkout HEAD submodule &&
+	git diff-files --quiet
+'
+
+test_done