acl: replace bare getpass.getuser() by platform function

Follows up dbadf28d4db0. bytestr() shouldn't be applied here because getuser()
isn't guaranteed to be all in ASCII.

This change means GetUserNameA() is used on Windows, but that's probably
better than trying to get the current user name in UNIX way.

hgext/acl.py

@@ -193,14 +193,11 b' 3) Deny access to a file to anyone but u'
 
 from __future__ import absolute_import
 
-import getpass
-
 from mercurial.i18n import _
 from mercurial import (
     error,
     extensions,
     match,
-    pycompat,
     registrar,
     util,
 )
@@ -341,7 +338,7 b' def hook(ui, repo, hooktype, node=None, '
             user = urlreq.unquote(url[3])
 
     if user is None:
-        user = pycompat.bytestr(getpass.getuser())
+        user = util.getuser()
 
     ui.debug('acl: checking access for user "%s"\n' % user)