Martin von Zweigbergk <martinvonz@google.com> [Thu, 14 Nov 2019 14:03:02 -0800] rev 43655
py3: use native string as fallback value for __file__ for consistency
This is not a bugfix (pycommpat.fsencode(b'') is a no-op on py3), but
the b'' value was inconsistent and confusing.
Differential Revision: https://phab.mercurial-scm.org/D7412