Add a scratch buffer to the history ring. (closes #3082)
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.
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