If no unmerged notebooks are present, nbmerge crashes

[tarmstrong]

Example run with a repository with one modified (not unmerged) notebook.

$ git status
# On branch master
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#   modified:   test.ipynb
#
no changes added to commit (use "git add" and/or "git commit -a")

$ nbmerge
Traceback (most recent call last):
  File "/home/tavish/envs/py27/bin/nbmerge", line 9, in <module>
    load_entry_point('nbdiff==0.0.0', 'console_scripts', 'nbmerge')()
  File "/home/tavish/capstone/nbdiff/nbdiff/commands.py", line 110, in merge
    nb_local = parser.parse(unmerged_notebooks[0][0])
IndexError: list index out of range