used in comparison check for notebook name change.
Unless the filenames are normalized,
unchanged names may result in false positives for a name change
(e.g. OS X uses NFD on the filesystem,
so u'\xfc' roundtripped to the filesystem will be u'u\u0308'),
which can result in the first save of a notebook after open performing the following actions:
1. save the recently opened notebook
2. `old_name != new_name`, so name change detected
3. delete old_name (which is actually new_name), which ultimately deletes the just-saved notebook
In master, this has a symptom of the first checkpoint failing because the first save actually deleted the file, and you can't checkpoint a notebook that doesn't exist.
closes#3360
MinRK
718bf61bd3