Add a check that ./dev.sh generate is never out of date.

[matanlurey]

Currently it's possible to update the code/file generators but not know they were not run in a commit.

The easiest way to do this would be to run ./dev.sh generate and use git to verify that the generator did not dirty any file, but it would be nice to bake this into ./dev.sh generate --check (or related) instead; that would require more instrumentation, though.