Pretest and branch on Friday June 19

[Chong Yidong]

Following discussion with Stefan, I plan to make the next pretest,
23.0.95, next Friday (June 19).  At that time, I will also cut the
EMACS_23_BASE branch and open the trunk for development.  If we need a
23.0.96 pretest, it will be made from the branch.

I'm not certain what version number to assign the trunk after branching.
I'm inclined to use 23.1.50, which assumes we'll use the trunk for
developing 23.2.  Maybe this is optimistic; as many will recall, what
ended up as Emacs 22 was originally slated for 21.3.  Opinions welcome.