prog: fix hints of data args

Hints for data args don't work.
We do all the work, but at the final stage we patch
arg in the _old_ program, not in the _new_ one.
So programs passed to the callback are all the same
and don't contain any mutations.
Tests did not catch this because they work right before that point
(don't test the actual interface function MutateWithHints).

Fix that and add a test that catches this.

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