If no cells are equal, diffing fails

[tarmstrong]

If you attempt to diff two completely different notebooks, you'll get an error like this:

(py27)tavish@temeraire:~/code/code-analysis (fourier)$ nbdiff IPythonReviewTime.ipynb FileOwnership.ipynb 
Traceback (most recent call last):
  File "/home/tavish/envs/py27/bin/nbdiff", line 9, in <module>
    load_entry_point('nbdiff==0.0.0', 'console_scripts', 'nbdiff')()
  File "/home/tavish/capstone/nbdiff/nbdiff/commands.py", line 52, in diff
    result = notebook_diff(notebook1, notebook2)
  File "/home/tavish/capstone/nbdiff/nbdiff/notebook_diff.py", line 11, in notebook_diff
    line_diffs = diff_modified_items(diffed_nb)
  File "/home/tavish/capstone/nbdiff/nbdiff/diff.py", line 58, in diff_modified_items
    cellslist[i]['modifiedvalue'].data["input"].splitlines(),
  File "/home/tavish/capstone/nbdiff/nbdiff/diff.py", line 9, in diff
    dps = diff_points(grid)
  File "/home/tavish/capstone/nbdiff/nbdiff/diff.py", line 67, in diff_points
    lcs_result = lcs(grid)
  File "/home/tavish/capstone/nbdiff/nbdiff/diff.py", line 127, in lcs
    highest = max(kcs.keys())
ValueError: max() arg is an empty sequence