Don't update changeset diffstat on changeset spec change

Created by: eseliger

The diff is not yet updated at this point and hence it caused confusion in the UI where the diffstat was out of sync with the actual diff. Since SetDerivedState is called after all code host operations and that one does SetDiffStat anyways, this just defers updating the diff stat of the changeset to after the commits have been pushed.

Closes https://github.com/sourcegraph/sourcegraph/issues/16196