Enable nbdiff without arguments to work from a hg repo

  Instead of invoking hg directly, I use python-hglib to
invoke hg commands.  This leads to cleaner code and better
control over error messages.  The only downside is that
this approach adds hglib as a new dependency.
make flake8 happier
make flake8 a little happier
remove hanging indent (flake8)
HgAdapter now works when nbdiff is called in any part of the repo tree
Don't try to diff unmerged notebooks
fixed mess with repo
make flake8 happy
correct orthographic error in comments
Make nbdiff work in environments in which hglib.open(path) doesn't return an error
Merge pull request #231 from tmbb/master

Make nbdiff without arguments work with a hg repo