Modified submodules are only warned about on PR creation

[kadiwa4]

rustbot gives a useful warning when someone modifies a submodule in their PR.

Accidental submodule modifications often happen while rebasing PRs onto a newer base commit (example 1, example 2). However, rustbot does not usually warn about that because the accidental modification warning is only sent in the greeting message and not if a submodule modification is introduced later on.