demandimport: explicitly declare `_session` at the module level

The `_session` module level variable is set within a function using the `global`
keyword. This confuses my `test-check-pyflakes.t`. Explicitly declaring the
variable at the top level solves the issue (and seems absolutely reasonable).

hgdemandimport/tracing.py

@@ -1,58 +1,59 b''
 # Support code for event tracing in Mercurial. Lives in demandimport
 # so it can also be used in demandimport.
 #
 # Copyright 2018 Google LLC.
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 from __future__ import absolute_import
 
 import contextlib
 import os
 
 _pipe = None
 _checked = False
+_session = 'none'
 
 def _isactive():
     global _pipe, _session, _checked
     if _pipe is None:
         if _checked:
             return False
         _checked = True
         if 'HGCATAPULTSERVERPIPE' not in os.environ:
             return False
         _pipe = open(os.environ['HGCATAPULTSERVERPIPE'], 'w', 1)
         _session = os.environ.get('HGCATAPULTSESSION', 'none')
     return True
 
 @contextlib.contextmanager
 def log(whencefmt, *whenceargs):
     if not _isactive():
         yield
         return
     whence = whencefmt % whenceargs
     try:
         # Both writes to the pipe are wrapped in try/except to ignore
         # errors, as we can see mysterious errors in here if the pager
         # is active. Presumably other conditions could trigger
         # problems too.
         try:
             _pipe.write('START %s %s\n' % (_session, whence))
         except IOError:
             pass
         yield
     finally:
         try:
             _pipe.write('END %s %s\n' % (_session, whence))
         except IOError:
             pass
 
 def counter(label, amount, *labelargs):
     if not _isactive():
         return
     l = label % labelargs
     # See above in log() for why this is in a try/except.
     try:
         _pipe.write('COUNTER %s %d %s\n' % (_session, amount, l))
     except IOError:
         pass