bug#15568: Lookup this-command-keys without a prefix argument

[Juri Linkov]

As discovered in bug#15200, (lookup-key isearch-mode-map (this-command-keys))
can't decide whether the key sequence is bound to an isearch command or not.

(lookup-key isearch-mode-map (kbd "C-u C-l")) => 1
(lookup-key isearch-mode-map (kbd "C-u C-w")) => 1

both return 1.

The length of universal-argument-num-events was used to remove
the prefix from this-command-keys to lookup a key without prefix,
but since universal-argument-num-events is gone it's unclear
what could replace it.