Echo overriding messages

[Jay Belanger]

Emacs used to not always cover up messages with keystroke
echoing.  For example, in Emacs 24.5, setting

(local-set-key "ab"
               (lambda ()
                 (interactive)
                 (message "Hello")
                 (let ((echo-keystrokes 0))
                   (push ?a unread-command-events))))

and then keying in "ab" would display
  Hello
in the minibuffer.  In the git master, it will display
  a-
Even setting `echo-keystrokes' to 0, the message is overwritten
and the echo area is blank (although the message does show up in the
*Messages* buffer). 

Parts of the Calc help mechanism rely on the old behavior.
Is the new behavior intended, or is there a simple way of
reproducing the old behavior?

Jay