Incorrect undo behavior following a code block #826

If i type this into bpython:
>>> for i in range(10):
... print(i)
And then do a rewind/undo after it prints the numbers 0-9, it removes only the 9. Pressing undo again undoes the rest.

I'm pretty sure this is due to the trailing blank lines in the buffer. So in curtsiesfrontend/repl.py, prompt_undo() sees the last input line as blank (self.history) even though the last output line (self.display_lines) is not blank. So it calls take_back_empty_line().

The trailing blank lines are definitely nice, so I think the goal behavior here is to undo the print and take you back to the buffer.

I might be able to fix this with the PR I'm working on right now, but I'm not exactly sure how yet, so I wanted to document it.