git add: do not add files from a submodule

It comes quite as a surprise to an unsuspecting Git user that calling
"git add submodule/file" (which is a mistake, alright) _removes_
the submodule in the index, and adds the file.  Instead, complain loudly.

While at it, be nice when the user said "git add submodule/" which is
most likely the consequence of tab-completion, and stage the submodule,
instead of trying to add the contents of that directory.

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

t/t7400-submodule-basic.sh

@@ -209,4 +209,29 @@ test_expect_success 'update --init' '
 
 '
 
+test_expect_success 'do not add files from a submodule' '
+
+	git reset --hard &&
+	test_must_fail git add init/a
+
+'
+
+test_expect_success 'gracefully add submodule with a trailing slash' '
+
+	git reset --hard &&
+	git commit -m "commit subproject" init &&
+	(cd init &&
+	 echo b > a) &&
+	git add init/ &&
+	git diff --exit-code --cached init &&
+	commit=$(cd init &&
+	 git commit -m update a >/dev/null &&
+	 git rev-parse HEAD) &&
+	git add init/ &&
+	test_must_fail git diff --exit-code --cached init &&
+	test $commit = $(git ls-files --stage |
+		sed -n "s/^160000 \([^ ]*\).*/\1/p")
+
+'
+
 test_done