Saving `*kakhist*` won't replace old history

kakhist-buf-save (*kak* key >) re-uses kakhist-save infrastructure, so it only affects un-saved and not-loaded commands (i.e. before last marker). This mostly defeats the purpose of editing *kakhist*. It should

1. rewrite the history file from scratch, possibly requiring confirmation
2. re-load history
3. re-create *kakhist* afterwards to match on-disk history.