~/.zshrc: Better escaping for the last-cmd prompt.

I think `!` is only used for history expansion, something I don't use
often and it is replaced by the command in question anyway. So that
should be ok?

.zshrc

	user="%{$fg[red]%}%n%{$reset_color%}"
	dir="%{$fg[cyan]%}%~%{$reset_color%}"
	lastcmd="$history[$(($HISTCMD-1))]"
	lastcmd_witherror="%(?.$lastcmd.%B$lastcmd%b)"
	lastcmd_witherror="%(?!$lastcmd!%B$lastcmd%b)"
	PROMPT="$user in $dir@$(repo_branch) $lastcmd_witherror
$(repo_char) $ "
	RPROMPT="%D{%Y-%m-%d} %{$fg[red]%}%t%{$reset_color%}"