hooks: say an existing hook still runs after enable, and restore it on uninstall

entire enable moved an existing hook to <hook>.pre-entire with only "Backed up existing pre-push to pre-push.pre-entire", which did not say the hook keeps running through Entire's chain. The message now says so and how to get it back, and the replace warning says plainly which hook keeps running. disable --uninstall reports each hook it restored. The README documents existing hooks and corrects what plain entire disable does; disable help lists post-rewrite.