largefiles: do not use platform.system()

See the previous patch for the reason.

Differential Revision: https://phab.mercurial-scm.org/D1020

hgext/largefiles/lfutil.py

@@ -12,7 +12,6 b' from __future__ import absolute_import'
 import copy
 import hashlib
 import os
-import platform
 import stat
 
 from mercurial.i18n import _
@@ -80,7 +79,7 b' def _usercachedir(ui):'
                         encoding.environ.get('APPDATA'))
         if appdata:
             return os.path.join(appdata, longname)
-    elif platform.system() == 'Darwin':
+    elif pycompat.sysplatform == 'darwin':
         home = encoding.environ.get('HOME')
         if home:
             return os.path.join(home, 'Library', 'Caches', longname)