When minibuffer.read is invoked without a default completion the first C-p will select the second to last
item where I think most people expect it to select the last item. 

This doesn't not appear to be the result of refactoring-completers branch or a new change.

Example:

Type "C-x b" then type part of a url that's not in the currently selected url and has multiple matches.
With no item selected if you press "C-p" it will select the second to last item. If you have more
completions than can be displayed on screen this might not be obvious as the last item might be off
screen so you may have to type "C-n" to see that you're on the second to last item.