pre-commit: Allow submodule check to be disabled

If hooks.submodule is 'false' disable all checks.  If an individual
hooks.<module>.update is 'true' then accept any update for that module.
Leave these options out of the hints printed.  Developers that know what
they are doing will be able to find them by reading the hook source.

pre-commit

@@ -175,16 +175,24 @@ short_commit() {
 }
 
 lookup_config_module_update() {
+	update=$(git config "hooks.$1.update")
+
+	# Special-case "true" to accept any update.
+	test "{$update}" = "{true}" && echo '.' && return
+
 	# Format is "aaaaaa..bbbbbb" for update aaaaaa => bbbbbb.
 	# Convert to regex "^aaaaaa[a-z0-9]* bbbbbb[a-z0-9]*$".
 	sha1ex='[a-z0-9][a-z0-9][a-z0-9][a-z0-9][a-z0-9][a-z0-9][a-z0-9]*'
 	regex='^\('"$sha1ex"'\)\.\.\('"$sha1ex"'\)$'
-	git config "hooks.$1.update" |
+	echo "$update" |
 	sed -n "/$regex/ {s/$regex/"'^\1[a-z0-9]* \2[a-z0-9]*$/;p;}' |
 	grep '.' # Return false if result is empty.
 }
 
 check_module() {
+	enabled=$(git config --get --bool hooks.submodule) || enabled=true
+	test "$enabled" = "false" && return
+
 	# Allow module-only commits without extra work.
 	test -z "$diffs_normal" && return