ignore changes in submodules when committing everything

covers `git commit -a` and `git citool` (`git add`, `git status`, etc).
they can still be added explicitly

.gitmodules

 [submodule "libmariadb"]
 	path = libmariadb
 	url = https://github.com/MariaDB/mariadb-connector-c
+	ignore = all
 [submodule "storage/rocksdb/rocksdb"]
 	path = storage/rocksdb/rocksdb
 	url = https://github.com/facebook/rocksdb.git
+	ignore = all