magit-rewrite-{set-commit-property,finish-step}: fix regressions

This is supposed to fix #1155. @dabrahams can you confirm that this works?

Make `magit-rewrite-set-commit-property` always return t so that the
`magit-section-action` hooks `magit-{apply,cherry-pick,revert}-hook`
run just the rewrite case, and not the default `commit` case too.

In `magit-rewrite-finish-step`, do not use `magit-cherry-pick-commit`.
That function does a refresh before we had a chance to call
`magit-rewrite-set-commit-property`.  If cherry-picking a commit fails,
then say so.