py3: do a fsdecode(), fsencode() dance in posix.py...
py3: do a fsdecode(), fsencode() dance in posix.py
We have to use `fsdecode()` instead of sysstr() because they are abusing
`PyUnicode_EncodeFSDefault()` to get back bytes.
Differential Revision:
https://phab.mercurial-scm.org/D5808