Increment HISTSIZE to fix hist_expire_dups_first

This fixes the old behavior which made it so all duplicates would be
deleted if the command history filled up with unique events.

> You should be sure to set the value of HISTSIZE to a larger number
> than SAVEHIST in order to give you some room for the duplicated
> events, otherwise this option will behave just like HIST_IGNORE_ALL_DUPS
> once the history fills up with unique events.