submodule: defend against submodule.update = !command in .gitmodules

In v2.15.4, we started to reject `submodule.update` settings in
`.gitmodules`. Let's raise a BUG if it somehow still made it through
from anywhere but the Git config.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>

builtin/submodule--helper.c

@@ -1482,6 +1482,8 @@ static void determine_submodule_update_strategy(struct repository *r,
 			die(_("Invalid update mode '%s' configured for submodule path '%s'"),
 				val, path);
 	} else if (sub->update_strategy.type != SM_UPDATE_UNSPECIFIED) {
+		if (sub->update_strategy.type == SM_UPDATE_COMMAND)
+			BUG("how did we read update = !command from .gitmodules?");
 		out->type = sub->update_strategy.type;
 		out->command = sub->update_strategy.command;
 	} else