tools/webpack-jumpstart: strengthen consistency check
tools/webpack-jumpstart: strengthen consistency check

Use the tree id recorded in the jumpstart cache as a much stronger
consistency check than simply comparing package-lock.json.  In case
--rebase wasn't specified, suggest it as a possible solution.

martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit