Allow to change input in history lookup.

Allow to change input in history lookup.

In previous version it does not work to lookup commands like ':r<up>' and
change the input after the history was initialized. This caused none working
lookup if the first attempt did not find any item. For example if ':foo<up>'
did not find any item and the user change the inputbox into ':o<up>' which
should match items, this retrieved also no item.

This patch fixes also none working lookup of set commands like ':set s<up>'.