As soon as we start processing a scrollback-push readline command, we
pause execution of all other readline commands until scrollback-push
retires. This means that we never get into a situation with two
active scrollback-push commands -- unless we are executing readline
commands via a script running "commandline -f":
since the first part of scrollback-push handling returns immediately,
the script will proceed before scrollback-push retires.
A second scrollback-push fails an assertion. Work around that for now.
In future, scrollback-push should block when invoked by such a script,
just like it does when invoked from bindings.
