It is easy for submodule changes to slip through during rebase (if
the developer uses the terrible `git add -u` command) and
for a maintainer to miss it (if they don't go over each change after
a rebase).
Protect against such mishaps by checking if a submodule was updated
(or .gitmodules itself was changes) and aborting the operation.
If the pull request title contains "submodule", assume the operation
was intended.
Allow bypassing the check with --allow-submodule.
Closesscylladb/scylladb#25418
6daa6178b1