t7406: submodule.<name>.update command must not be run from .gitmodules

submodule.<name>.update can be assigned an arbitrary command via setting
it to "!command". When this command is found in the regular config, Git
ought to just run that command instead of other update mechanisms.

However if that command is just found in the .gitmodules file, it is
potentially untrusted, which is why we do not run it.  Add a test
confirming the behavior.

Suggested-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7406-submodule-update.sh

@@ -406,6 +406,14 @@ test_expect_success 'submodule update - command in .git/config' '
 	)
 '
 
+test_expect_success 'submodule update - command in .gitmodules is ignored' '
+	test_when_finished "git -C super reset --hard HEAD^" &&
+	git -C super config -f .gitmodules submodule.submodule.update "!false" &&
+	git -C super commit -a -m "add command to .gitmodules file" &&
+	git -C super/submodule reset --hard $submodulesha1^ &&
+	git -C super submodule update submodule
+'
+
 cat << EOF >expect
 Execution of 'false $submodulesha1' failed in submodule path 'submodule'
 EOF