submodule--helper: teach config subcommand --unset

This teaches submodule--helper config the --unset option, which removes
the specified configuration key from the .gitmodule file.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t7411-submodule-config.sh b/t/t7411-submodule-config.sh
index 89690b7..fcc0fb8 100755
--- a/t/t7411-submodule-config.sh
+++ b/t/t7411-submodule-config.sh
@@ -142,6 +142,15 @@
 	)
 '
 
+test_expect_success 'unsetting submodules config from the working tree with "submodule--helper config --unset"' '
+	(cd super &&
+		git submodule--helper config --unset submodule.submodule.url &&
+		git submodule--helper config submodule.submodule.url >actual &&
+		test_must_be_empty actual
+	)
+'
+
+
 test_expect_success 'writing submodules config with "submodule--helper config"' '
 	(cd super &&
 		echo "new_url" >expect &&