This patch creates an extra slot in the history ring for a scratch
buffer (at h->last).  If you are editing inside that buffer, it is
preserved when you scroll up/down through the history.  Editing while in
other places in history are *not* preserved with this patch.

Another behavior change worth noting with this patch: the position in
history is now reset to the scratch buffer after each input entry.
Before, the position would be stay wherever it was - you didn't restart
at the "bottom" each time.