windows: if username(uid=None) is loaded, just use getpass

This is at least consistent with what we do on other platforms in the
base case. I don't know enough about Windows to fill in other cases
that might exist here, but this at least should be a start.

Differential Revision: https://phab.mercurial-scm.org/D7679

mercurial/windows.py

@@ -8,6 +8,7 b''
 from __future__ import absolute_import
 
 import errno
+import getpass
 import msvcrt
 import os
 import re
@@ -563,6 +564,8 b' def username(uid=None):'
     """Return the name of the user with the given uid.
 
     If uid is None, return the name of the current user."""
+    if not uid:
+        return pycompat.fsencode(getpass.getuser())
     return None