bug#13255: 24.2.50; output in nonvisible shell buffer changes prompt

[Chong Yidong]

Stephen Berman <stephen.berman@gmx.net> writes:

>> emacs -Q -f shell
>>
>> ;; run this in shell buffer
>> while : ; do
>>  sleep 0.5
>>  echo hi
>> done
>>
>> C-x b *scratch* RET
>> C-h
>
> I also get no prompt on typing `C-h'; but if I uncheck "Blink Cursor"
> in the Options menu and again type `C-h', then I see exactly what rms
> reported.

I've made a commit to trunk which should fix this.