ChangeLog.3 in master

[Paul Eggert <eggert@cs.ucla.edu>]

I merged emacs-25 into master again today. This time, I attempted to 
cleanup the ChangeLog.2 mess in master, by creating a file ChangeLog.3 
(only in master) which attempts to keep log entries for the master 
branch. The emacs-25 branch does not have a ChangeLog.3 file. The master 
and emacs-25 branches should have identical ChangeLog.2 files 
immediately after merging emacs-25 into master, because there should be 
no reason to change ChangeLog.2 in master. (I hope this is all clear. :-)

Undoubtedly there will be further problems in this area, as the 
ChangeLog stuff is still way too awkward. I'm mildly inclined to stop 
updating ChangeLog files in the repository, and ask developers to use 
something like 'make ChangeLog' if they want more-recent entries. I 
realize not everyone agrees; on the other hand, ChangeLog.2 was becoming 
fairly confusing in the master branch and ChangeLog.3 is not that much 
better.