Don't run a code cell twice

[lgpage]

A cell's code is pointlessly re-run again via ipython to get the "<ipython-input-{0}-{1}>" f_code.co_filename set and the code cached.

I don't know enough, yet, about the IPython API to do this better inside the debugger

https://github.com/lgpage/nbtutor/blob/master/nbtutor/ipython/magic.py#L66