Make the unread events replace the last command and echo.
Used in `universal-argument-other-key'.
`universal-argument-other-key' rereads the event just typed.
It then gets translated through `function-key-map'.
The translated event has to replace the real events,
both in the value of (this-command-keys) and in echoing.
To achieve this, `universal-argument-other-key' calls
`reset-this-command-lengths', which discards the record of reading
these events the first time.
(fn)