test:changed --land: dist counts as fresh when it predates the rebase #591

The land gate decides whether to rebuild packages/ui/dist and site/dist by comparing their mtime with the files in the PR diff. A rebase brings in package sources the diff does not list, so a dist built hours earlier (before the rebase) is judged fresh and the site build fails (MISSING_EXPORT ChartScope on #579) or tests run against old code. In land mode the gate must always rebuild both (it already budgets for it).