Fix ImportDB.get_default when CWD is the root directory (PyInf#13677)

[saharan-deshaw]

The enhancements made in commit 7272bdb (Issue #346) introduced a bug where ImportDB.get_default(None) fails when the current working directory is the root directory. Reproducer:

>>> from pyflyby._importdb import ImportDB 
>>> os.chdir('/')
>>> db = ImportDB.get_default(None)
ValueError: No know path are safe

The reason is that in this case, there are no parent directories to check. So the logic incorrectly thinks that there is no safe path.

[sac111gp]

Hi team, any ETA on this?
We'd like to get this fixed soon since we are planning an upgrade of pyflyby in 1 week time.

[saharan-deshaw]

@Carreau, any updates here?

[sac111gp]

ping.