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

mercurial/utils/procutil.py

@@ -289,7 +289,7 b' def hgexecutable():'
         elif (
             not pycompat.iswindows
             and os.path.basename(
-                pycompat.fsencode(getattr(mainmod, '__file__', b''))
+                pycompat.fsencode(getattr(mainmod, '__file__', ''))
             )
             == b'hg'
         ):