REPL: fix for history navigation
We were using a comint-get-old-input function that was including the
prompt in its returned value. This was no problem most of the time
because we don't use comint-send-input before the process mark, but
there's another circumstance under which comint-get-old-input is
called, namely, when reaching the end of the input history. When
history is exhausted, the "old input" is inserted (go figure), and we
were inserting a prompt, wreaking havoc with its read-only-ness.